Here's an incomplete and very minimal demonstration of the kind of thing you might do, which is still rather time-consuming to achieve for all its abject failure to fulfil the desiderata.
The following code creates a fairly minimal pic
called brush stroke
. It accepts a list of key-value options. The options implemented below are colour
, text
and name
.
The pic
creates the node and then scales a scoped and filled path based on a stroke vector from Lazur URH's work at https://openclipart.org/detail/211152/brush-stroke-2, extracted and exported to TikZ code using Inkscape. I edited this code a bit and turned the filling colour into a variable which picks up the colour
option and makes it partially transparent.
\documentclass[border=10pt,multi,tikz]{standalone}
\usetikzlibrary{backgrounds}
\makeatletter
\tikzset{%
pics/brush stroke/.style={%
code={%
\tikzset{%
brushes/.cd,
#1,
}
\node (\brushes@name) [anchor=west, pic actions] {\brushes@text};
\pgfmathsetmacro\brushes@textwidth{((width("\brushes@text"))+40pt)/112mm}
\pgfmathsetmacro\brushes@texttotalheight{((height("\brushes@text")+depth("\brushes@text"))+10pt)/15mm}
% based on stroke from Lazur URH's work at https://openclipart.org/detail/211152/brush-stroke-2, extracted and exported using Inkscape
\begin{scope}[on background layer, y=0.8pt, x=0.8pt, xscale=\brushes@textwidth, yscale=\brushes@texttotalheight, yshift=-100+5pt, xshift=-10pt]
\path [fill=\brushes@colour, fill opacity=.2, pic actions] (2.5,85.29) .. controls (1.1193,85.29) and (0,86.4093) .. (0,87.79) .. controls (0,89.1707) and (1.1193,90.29) .. (2.5,90.29) .. controls (72.08,90.29) and (316.55,88.4202) .. (400,87.79) .. controls (316.548,87.1598) and (72.08,85.29) .. (2.5,85.29) -- cycle(2.5,88.8614) .. controls (1.1193,88.8614) and (0,89.9807) .. (0,91.3614) .. controls (0,92.7421) and (1.1193,93.8614) .. (2.5,93.8614) .. controls (72.08,93.8614) and (316.55,91.9916) .. (400,91.3614) .. controls (316.548,90.7312) and (72.08,88.8614) .. (2.5,88.8614) -- cycle(2.5,92.4328) .. controls (1.1193,92.4328) and (0,93.5521) .. (0,94.9328) .. controls (0,96.3135) and (1.1193,97.4328) .. (2.5,97.4328) .. controls (72.08,97.4328) and (316.55,95.563) .. (400,94.9328) .. controls (316.548,94.3026) and (72.08,92.4328) .. (2.5,92.4328) -- cycle(2.5,96.0042) .. controls (1.1193,96.0042) and (0,97.1235) .. (0,98.5042) .. controls (0,99.8849) and (1.1193,101.0042) .. (2.5,101.0042) .. controls (72.08,101.0042) and (316.55,99.1344) .. (400,98.5042) .. controls (316.548,97.874) and (72.08,96.0042) .. (2.5,96.0042) -- cycle(2.5,99.5756) .. controls (1.1193,99.5756) and (0,100.6949) .. (0,102.0756) .. controls (0,103.4563) and (1.1193,104.5756) .. (2.5,104.5756) .. controls (72.08,104.5756) and (316.55,102.7058) .. (400,102.0756) .. controls (316.548,101.4454) and (72.08,99.5756) .. (2.5,99.5756) -- cycle(2.5,103.147) .. controls (1.1193,103.147) and (0,104.2663) .. (0,105.647) .. controls (0,107.0277) and (1.1193,108.147) .. (2.5,108.147) .. controls (72.08,108.147) and (316.55,106.2772) .. (400,105.647) .. controls (316.548,105.0168) and (72.08,103.147) .. (2.5,103.147) -- cycle(2.5,106.7184) .. controls (1.1193,106.7184) and (0,107.8377) .. (0,109.2184) .. controls (0,110.5991) and (1.1193,111.7184) .. (2.5,111.7184) .. controls (72.08,111.7184) and (316.55,109.8486) .. (400,109.2184) .. controls (316.548,108.5882) and (72.08,106.7184) .. (2.5,106.7184) -- cycle(2.5,110.2898) .. controls (1.1193,110.2898) and (0,111.4091) .. (0,112.7898) .. controls (0,114.1705) and (1.1193,115.2898) .. (2.5,115.2898) .. controls (72.08,115.2898) and (316.55,113.42) .. (400,112.7898) .. controls (316.548,112.1596) and (72.08,110.2898) .. (2.5,110.2898) -- cycle(2.5,113.8612) .. controls (1.1193,113.8612) and (0,114.9805) .. (0,116.3612) .. controls (0,117.7419) and (1.1193,118.8612) .. (2.5,118.8612) .. controls (72.08,118.8612) and (316.55,116.9914) .. (400,116.3612) .. controls (316.548,115.731) and (72.08,113.8612) .. (2.5,113.8612) -- cycle(2.5,117.4326) .. controls (1.1193,117.4326) and (0,118.5519) .. (0,119.9326) .. controls (0,121.3133) and (1.1193,122.4326) .. (2.5,122.4326) .. controls (72.08,122.4326) and (316.55,120.5628) .. (400,119.9326) .. controls (316.548,119.3024) and (72.08,117.4326) .. (2.5,117.4326) -- cycle(2.5,121.004) .. controls (1.1193,121.004) and (0,122.1233) .. (0,123.504) .. controls (0,124.8847) and (1.1193,126.004) .. (2.5,126.004) .. controls (72.08,126.004) and (316.55,124.1342) .. (400,123.504) .. controls (316.548,122.8738) and (72.08,121.004) .. (2.5,121.004) -- cycle(2.5,124.5754) .. controls (1.1193,124.5754) and (0,125.6947) .. (0,127.0754) .. controls (0,128.4561) and (1.1193,129.5754) .. (2.5,129.5754) .. controls (72.08,129.5754) and (316.55,127.7056) .. (400,127.0754) .. controls (316.548,126.4452) and (72.08,124.5754) .. (2.5,124.5754) -- cycle(2.5,128.1468) .. controls (1.1193,128.1468) and (0,129.2661) .. (0,130.6468) .. controls (0,132.0275) and (1.1193,133.1468) .. (2.5,133.1468) .. controls (72.08,133.1468) and (316.55,131.277) .. (400,130.6468) .. controls (316.548,130.0166) and (72.08,128.1468) .. (2.5,128.1468) -- cycle(2.5,131.7182) .. controls (1.1193,131.7182) and (0,132.8375) .. (0,134.2182) .. controls (0,135.5989) and (1.1193,136.7182) .. (2.5,136.7182) .. controls (72.08,136.7182) and (316.55,134.8484) .. (400,134.2182) .. controls (316.548,133.588) and (72.08,131.7182) .. (2.5,131.7182) -- cycle(2.5,135.2896) .. controls (1.1193,135.2896) and (0,136.4089) .. (0,137.7896) .. controls (0,139.1703) and (1.1193,140.2896) .. (2.5,140.2896) .. controls (72.08,140.2896) and (316.55,138.4198) .. (400,137.7896) .. controls (316.548,137.1594) and (72.08,135.2896) .. (2.5,135.2896) -- cycle;
\end{scope}
},
},
brushes/.search also={/tikz},
brushes/.cd,
name/.store in=\brushes@name,
colour/.store in=\brushes@colour,
text/.store in=\brushes@text,
name=brush node,
colour=green,
text=,
}
\makeatother
\begin{document}
\begin{tikzpicture}
\pic at (0,0) {brush stroke={text=This is some text.}};
\pic [font=\sffamily\large] at (0,1) {brush stroke={text=This is some more text., colour=blue, blue}};
\end{tikzpicture}
\end{document}
The rather pedestrian result:

EDIT
Here's a rather poorly coordinated example, just for demonstration, which combines the above with customised arrows and the pencil draw
decoration from Claudio Fiandrino's answer.

\documentclass[border=10pt,multi,tikz]{standalone}
\usetikzlibrary{backgrounds,decorations.pathmorphing,fit,arrows.meta,bending}
\tikzset{pencil draw/.style={% code from Claudio Fiandrino's answer at https://tex.stackexchange.com/a/95333/
draw,
decorate,
decoration={random steps, segment length=3mm, amplitude=0.2mm}
},
}
\makeatletter
\tikzset{%
pics/brush stroke/.style={%
code={%
\tikzset{%
brushes/.cd,
#1,
}
\begin{scope}[local bounding box/.expanded=\brushes@name]
\node (\brushes@name-text) [anchor=west, pic actions] {\brushes@text};
\pgfmathsetmacro\brushes@textwidth{((width("\brushes@text"))+40pt)/112mm}
\pgfmathsetmacro\brushes@texttotalheight{((height("\brushes@text")+depth("\brushes@text"))+10pt)/15mm}
% based on stroke from Lazur URH's work at https://openclipart.org/detail/211152/brush-stroke-2, extracted and exported using Inkscape
\begin{scope}[on background layer, y=0.8pt, x=0.8pt, xscale=\brushes@textwidth, yscale=\brushes@texttotalheight, yshift=-100+5pt, xshift=-10pt]
\path [fill=\brushes@colour, fill opacity=.2, pic actions] (2.5,85.29) .. controls (1.1193,85.29) and (0,86.4093) .. (0,87.79) .. controls (0,89.1707) and (1.1193,90.29) .. (2.5,90.29) .. controls (72.08,90.29) and (316.55,88.4202) .. (400,87.79) .. controls (316.548,87.1598) and (72.08,85.29) .. (2.5,85.29) -- cycle(2.5,88.8614) .. controls (1.1193,88.8614) and (0,89.9807) .. (0,91.3614) .. controls (0,92.7421) and (1.1193,93.8614) .. (2.5,93.8614) .. controls (72.08,93.8614) and (316.55,91.9916) .. (400,91.3614) .. controls (316.548,90.7312) and (72.08,88.8614) .. (2.5,88.8614) -- cycle(2.5,92.4328) .. controls (1.1193,92.4328) and (0,93.5521) .. (0,94.9328) .. controls (0,96.3135) and (1.1193,97.4328) .. (2.5,97.4328) .. controls (72.08,97.4328) and (316.55,95.563) .. (400,94.9328) .. controls (316.548,94.3026) and (72.08,92.4328) .. (2.5,92.4328) -- cycle(2.5,96.0042) .. controls (1.1193,96.0042) and (0,97.1235) .. (0,98.5042) .. controls (0,99.8849) and (1.1193,101.0042) .. (2.5,101.0042) .. controls (72.08,101.0042) and (316.55,99.1344) .. (400,98.5042) .. controls (316.548,97.874) and (72.08,96.0042) .. (2.5,96.0042) -- cycle(2.5,99.5756) .. controls (1.1193,99.5756) and (0,100.6949) .. (0,102.0756) .. controls (0,103.4563) and (1.1193,104.5756) .. (2.5,104.5756) .. controls (72.08,104.5756) and (316.55,102.7058) .. (400,102.0756) .. controls (316.548,101.4454) and (72.08,99.5756) .. (2.5,99.5756) -- cycle(2.5,103.147) .. controls (1.1193,103.147) and (0,104.2663) .. (0,105.647) .. controls (0,107.0277) and (1.1193,108.147) .. (2.5,108.147) .. controls (72.08,108.147) and (316.55,106.2772) .. (400,105.647) .. controls (316.548,105.0168) and (72.08,103.147) .. (2.5,103.147) -- cycle(2.5,106.7184) .. controls (1.1193,106.7184) and (0,107.8377) .. (0,109.2184) .. controls (0,110.5991) and (1.1193,111.7184) .. (2.5,111.7184) .. controls (72.08,111.7184) and (316.55,109.8486) .. (400,109.2184) .. controls (316.548,108.5882) and (72.08,106.7184) .. (2.5,106.7184) -- cycle(2.5,110.2898) .. controls (1.1193,110.2898) and (0,111.4091) .. (0,112.7898) .. controls (0,114.1705) and (1.1193,115.2898) .. (2.5,115.2898) .. controls (72.08,115.2898) and (316.55,113.42) .. (400,112.7898) .. controls (316.548,112.1596) and (72.08,110.2898) .. (2.5,110.2898) -- cycle(2.5,113.8612) .. controls (1.1193,113.8612) and (0,114.9805) .. (0,116.3612) .. controls (0,117.7419) and (1.1193,118.8612) .. (2.5,118.8612) .. controls (72.08,118.8612) and (316.55,116.9914) .. (400,116.3612) .. controls (316.548,115.731) and (72.08,113.8612) .. (2.5,113.8612) -- cycle(2.5,117.4326) .. controls (1.1193,117.4326) and (0,118.5519) .. (0,119.9326) .. controls (0,121.3133) and (1.1193,122.4326) .. (2.5,122.4326) .. controls (72.08,122.4326) and (316.55,120.5628) .. (400,119.9326) .. controls (316.548,119.3024) and (72.08,117.4326) .. (2.5,117.4326) -- cycle(2.5,121.004) .. controls (1.1193,121.004) and (0,122.1233) .. (0,123.504) .. controls (0,124.8847) and (1.1193,126.004) .. (2.5,126.004) .. controls (72.08,126.004) and (316.55,124.1342) .. (400,123.504) .. controls (316.548,122.8738) and (72.08,121.004) .. (2.5,121.004) -- cycle(2.5,124.5754) .. controls (1.1193,124.5754) and (0,125.6947) .. (0,127.0754) .. controls (0,128.4561) and (1.1193,129.5754) .. (2.5,129.5754) .. controls (72.08,129.5754) and (316.55,127.7056) .. (400,127.0754) .. controls (316.548,126.4452) and (72.08,124.5754) .. (2.5,124.5754) -- cycle(2.5,128.1468) .. controls (1.1193,128.1468) and (0,129.2661) .. (0,130.6468) .. controls (0,132.0275) and (1.1193,133.1468) .. (2.5,133.1468) .. controls (72.08,133.1468) and (316.55,131.277) .. (400,130.6468) .. controls (316.548,130.0166) and (72.08,128.1468) .. (2.5,128.1468) -- cycle(2.5,131.7182) .. controls (1.1193,131.7182) and (0,132.8375) .. (0,134.2182) .. controls (0,135.5989) and (1.1193,136.7182) .. (2.5,136.7182) .. controls (72.08,136.7182) and (316.55,134.8484) .. (400,134.2182) .. controls (316.548,133.588) and (72.08,131.7182) .. (2.5,131.7182) -- cycle(2.5,135.2896) .. controls (1.1193,135.2896) and (0,136.4089) .. (0,137.7896) .. controls (0,139.1703) and (1.1193,140.2896) .. (2.5,140.2896) .. controls (72.08,140.2896) and (316.55,138.4198) .. (400,137.7896) .. controls (316.548,137.1594) and (72.08,135.2896) .. (2.5,135.2896) -- cycle;
\end{scope}
\end{scope}
},
},
brushes/.search also={/tikz},
brushes/.cd,
name/.store in=\brushes@name,
colour/.store in=\brushes@colour,
text/.store in=\brushes@text,
name=brush node,
colour=green,
text=,
/tikz/.cd,
thin pencil/.style={draw=#1!75!gray, pencil draw},
thick pencil/.style={draw=#1!75!gray!50, pencil draw, line width=1pt},
thin arrow/.style={draw=#1!75!gray, >={Triangle[bend]}},
thick arrow/.style={draw=#1!75!gray, >={Triangle[bend]}, line width=1pt},
}
\makeatother
\begin{document}
\begin{tikzpicture}
\pic at (0,0) {brush stroke={name=this, text=This is some text.}};
\pic [font=\sffamily\large] at (0,1) {brush stroke={name=that, text=This is some more text., colour=blue, blue}};
\node [thick pencil=blue, fit=(that)] {};
\node [thick pencil=green, fit=(this)] {};
\draw [thin arrow=blue, ->] (that-text.east) [bend left=45] to ++(.5,-.75) node [thin pencil=blue, anchor=north] {$c^2$};
\draw [thick arrow=magenta, <->] ([xshift=3pt]this-text.west) [out=180, in=180] to ([xshift=3pt]that-text.west);
\node [fit=(current bounding box), pencil draw, line width=1pt, double=gray!10, draw=gray] {};
\end{tikzpicture}
\end{document}
EDIT EDIT
Here is a version which this time uses code from Alain Matthes's answer for the hand-hewn look. This kind-of-partially-supports-rounding-if-you-are-very-careful-and-like-to-live-dangerously. (If you get too large dimension errors, you've exceeded TikZ's limits. Unsurprisingly, this can vary randomly. Hence, success on one run does not guarantee success on the next with the same code!)
\documentclass[border=10pt,multi,tikz]{standalone}
\usetikzlibrary{backgrounds,decorations.pathmorphing,fit,arrows.meta,bending}
\makeatletter
\tikzset{%
pics/brush stroke/.style={%
code={%
\tikzset{%
brushes/.cd,
#1,
}
\begin{scope}[local bounding box/.expanded=\brushes@name]
\node (\brushes@name-text) [anchor=west, pic actions] {\brushes@text};
\pgfmathsetmacro\brushes@textwidth{((width("\brushes@text"))+40pt)/112mm}
\pgfmathsetmacro\brushes@texttotalheight{((height("\brushes@text")+depth("\brushes@text"))+10pt)/15mm}
% based on stroke from Lazur URH's work at https://openclipart.org/detail/211152/brush-stroke-2, extracted and exported using Inkscape
\begin{scope}[on background layer, y=0.8pt, x=0.8pt, xscale=\brushes@textwidth, yscale=\brushes@texttotalheight, yshift=-100+5pt, xshift=-10pt]
\path [fill=\brushes@colour, fill opacity=.2, pic actions] (2.5,85.29) .. controls (1.1193,85.29) and (0,86.4093) .. (0,87.79) .. controls (0,89.1707) and (1.1193,90.29) .. (2.5,90.29) .. controls (72.08,90.29) and (316.55,88.4202) .. (400,87.79) .. controls (316.548,87.1598) and (72.08,85.29) .. (2.5,85.29) -- cycle(2.5,88.8614) .. controls (1.1193,88.8614) and (0,89.9807) .. (0,91.3614) .. controls (0,92.7421) and (1.1193,93.8614) .. (2.5,93.8614) .. controls (72.08,93.8614) and (316.55,91.9916) .. (400,91.3614) .. controls (316.548,90.7312) and (72.08,88.8614) .. (2.5,88.8614) -- cycle(2.5,92.4328) .. controls (1.1193,92.4328) and (0,93.5521) .. (0,94.9328) .. controls (0,96.3135) and (1.1193,97.4328) .. (2.5,97.4328) .. controls (72.08,97.4328) and (316.55,95.563) .. (400,94.9328) .. controls (316.548,94.3026) and (72.08,92.4328) .. (2.5,92.4328) -- cycle(2.5,96.0042) .. controls (1.1193,96.0042) and (0,97.1235) .. (0,98.5042) .. controls (0,99.8849) and (1.1193,101.0042) .. (2.5,101.0042) .. controls (72.08,101.0042) and (316.55,99.1344) .. (400,98.5042) .. controls (316.548,97.874) and (72.08,96.0042) .. (2.5,96.0042) -- cycle(2.5,99.5756) .. controls (1.1193,99.5756) and (0,100.6949) .. (0,102.0756) .. controls (0,103.4563) and (1.1193,104.5756) .. (2.5,104.5756) .. controls (72.08,104.5756) and (316.55,102.7058) .. (400,102.0756) .. controls (316.548,101.4454) and (72.08,99.5756) .. (2.5,99.5756) -- cycle(2.5,103.147) .. controls (1.1193,103.147) and (0,104.2663) .. (0,105.647) .. controls (0,107.0277) and (1.1193,108.147) .. (2.5,108.147) .. controls (72.08,108.147) and (316.55,106.2772) .. (400,105.647) .. controls (316.548,105.0168) and (72.08,103.147) .. (2.5,103.147) -- cycle(2.5,106.7184) .. controls (1.1193,106.7184) and (0,107.8377) .. (0,109.2184) .. controls (0,110.5991) and (1.1193,111.7184) .. (2.5,111.7184) .. controls (72.08,111.7184) and (316.55,109.8486) .. (400,109.2184) .. controls (316.548,108.5882) and (72.08,106.7184) .. (2.5,106.7184) -- cycle(2.5,110.2898) .. controls (1.1193,110.2898) and (0,111.4091) .. (0,112.7898) .. controls (0,114.1705) and (1.1193,115.2898) .. (2.5,115.2898) .. controls (72.08,115.2898) and (316.55,113.42) .. (400,112.7898) .. controls (316.548,112.1596) and (72.08,110.2898) .. (2.5,110.2898) -- cycle(2.5,113.8612) .. controls (1.1193,113.8612) and (0,114.9805) .. (0,116.3612) .. controls (0,117.7419) and (1.1193,118.8612) .. (2.5,118.8612) .. controls (72.08,118.8612) and (316.55,116.9914) .. (400,116.3612) .. controls (316.548,115.731) and (72.08,113.8612) .. (2.5,113.8612) -- cycle(2.5,117.4326) .. controls (1.1193,117.4326) and (0,118.5519) .. (0,119.9326) .. controls (0,121.3133) and (1.1193,122.4326) .. (2.5,122.4326) .. controls (72.08,122.4326) and (316.55,120.5628) .. (400,119.9326) .. controls (316.548,119.3024) and (72.08,117.4326) .. (2.5,117.4326) -- cycle(2.5,121.004) .. controls (1.1193,121.004) and (0,122.1233) .. (0,123.504) .. controls (0,124.8847) and (1.1193,126.004) .. (2.5,126.004) .. controls (72.08,126.004) and (316.55,124.1342) .. (400,123.504) .. controls (316.548,122.8738) and (72.08,121.004) .. (2.5,121.004) -- cycle(2.5,124.5754) .. controls (1.1193,124.5754) and (0,125.6947) .. (0,127.0754) .. controls (0,128.4561) and (1.1193,129.5754) .. (2.5,129.5754) .. controls (72.08,129.5754) and (316.55,127.7056) .. (400,127.0754) .. controls (316.548,126.4452) and (72.08,124.5754) .. (2.5,124.5754) -- cycle(2.5,128.1468) .. controls (1.1193,128.1468) and (0,129.2661) .. (0,130.6468) .. controls (0,132.0275) and (1.1193,133.1468) .. (2.5,133.1468) .. controls (72.08,133.1468) and (316.55,131.277) .. (400,130.6468) .. controls (316.548,130.0166) and (72.08,128.1468) .. (2.5,128.1468) -- cycle(2.5,131.7182) .. controls (1.1193,131.7182) and (0,132.8375) .. (0,134.2182) .. controls (0,135.5989) and (1.1193,136.7182) .. (2.5,136.7182) .. controls (72.08,136.7182) and (316.55,134.8484) .. (400,134.2182) .. controls (316.548,133.588) and (72.08,131.7182) .. (2.5,131.7182) -- cycle(2.5,135.2896) .. controls (1.1193,135.2896) and (0,136.4089) .. (0,137.7896) .. controls (0,139.1703) and (1.1193,140.2896) .. (2.5,140.2896) .. controls (72.08,140.2896) and (316.55,138.4198) .. (400,137.7896) .. controls (316.548,137.1594) and (72.08,135.2896) .. (2.5,135.2896) -- cycle;
\end{scope}
\end{scope}
},
},
brushes/.search also={/tikz},
brushes/.cd,
name/.store in=\brushes@name,
colour/.store in=\brushes@colour,
text/.store in=\brushes@text,
name=brush node,
colour=green,
text=,
}
\makeatother
% code from Alain Matthes's answer at https://tex.stackexchange.com/a/49272/
\pgfdeclaredecoration{free hand}{start}
{
\state{start}[width = +0pt,
next state=step,
persistent precomputation = \pgfdecoratepathhascornerstrue]{}
\state{step}[auto end on length = 3pt,
auto corner on length = 3pt,
width=+2pt]
{
\pgfpathlineto{
\pgfpointadd
{\pgfpoint{2pt}{0pt}}
{\pgfpoint{rand*0.15pt}{rand*0.15pt}}
}
}
\state{final}
{}
}
\tikzset{free hand/.style={
decorate,
decoration={free hand}
}
}
\newcommand\freedraw{\draw [free hand]}
\tikzset{%
thin arrow/.style={draw=#1!75!gray, >={Triangle[bend]}},
thick arrow/.style={draw=#1!75!gray, >={Triangle[bend]}, line width=1pt},
thin free hand/.style={draw=#1!75!gray, free hand},
thick free hand/.style={draw=#1!71!gray!50, free hand, line width=1pt},
}
\begin{document}
\begin{tikzpicture}
\pic at (0,0) {brush stroke={name=this, text=This is some text.}};
\pic [font=\sffamily\large] at (0,1) {brush stroke={name=that, text=This is some more text., colour=blue, blue}};
\node [thick free hand=blue, fit=(that)] {};
\node [thick free hand=green, fit=(this)] {};
\draw [thin arrow=blue, ->] (that-text.east) [bend left=45] to ++(.5,-.75) node [thin free hand=blue, anchor=north] {$c^2$};
\draw [thick arrow=magenta, <->] ([xshift=3pt]this-text.west) [out=180, in=180] to ([xshift=3pt]that-text.west);
\node [fit=(current bounding box), rounded corners=5pt, free hand, line width=1.5pt, draw=gray, postaction={draw=gray!10, line width=.75pt, opacity=.5}] {};
\end{tikzpicture}
\end{document}

See the answers to this question (of which Alain's is just one of many excellent ones) for a variety of approaches to creating that elusive 'hand-hewn' look with LaTeX. Not all of the answers use TikZ, but many do.
Note that Marc van Dongen's answer shows the pencil draw
approach applied to arcs. The issue with rounding the corners of rectangles is that the arcs wanted are very small and this creates various kinds of complexities which cause no end of trouble.