I can't answer your question about any packages that do that. Some of those parameters could be changed. For example, if you alway use \( ... \)
instead of $...$
, then you could make $
just be a normal punctuation symbol (or you could use \$
). Likewise, \&
, \_
and \^
could easily be swapped with their nonslashed counterparts. There's no need for ~
to be active, you could easily define \~
to have the same effect. As Will notes in the comments, #
can be made a symbol.
I wasn't able to get comments working. I can't figure out how to get a macro to expand to an end of line token. Actually, I'm not even sure that's one of the tokens that gets to TeX's "stomach."
The real problem would come with braces. You really do need two additional characters to act as groupings. You could make two characters you don't frequently use be those.
So barring braces being special and not having line comments (although it'd be easy to define a block comment like \def\%#1\relax{}
which might be sufficient for your purposes), this seems to work.
\newenvironment{plaintext}{%
\catcode`\$12
\def\&{&}%
\catcode`\&12
\def\_{_}%
\catcode`\_12
\def\^{^}%
\catcode`\^12
\catcode`\#12
\catcode`\%12
\let\~~%
\catcode`\~12
}{}
Use it like
\begin{plaintext}
Here is some test text % ^ & _ $ # &.
How about some math \(x\_y\^z\). You're still out of luck with braces
though.
\end{plaintext}