We may notice that the numerator and denominator in the following fraction are not aligned. And, to me it does not look good.
How to align the numerator and denominator?
\documentclass{article}
\usepackage{amsmath,mathrsfs,xcolor,mathtools}
\let\xoverline\overline % supply your definition
\begin{document}
\begin{equation}
\dfrac{1-k^{m+n+d}}{1-k}
\end{equation}
\end{document}
The above yields:
\frac{1-k^{m+n+d}}{1-k^{\hphantom{m+n+d}}}
\dfrac{1-k^{m+n+d}}{1-k\quad}
But I'm not sure it looks nicer than the default one.