I seem to often run into code with what I call random indentation.
Because it is in source control, it's a can of worm to re-indent everything and submit the code properly formatted (just makes it harder to look at history afterward, and it doesn't help when people mess the formatting again after...)
(let ((non-persistent-formatting t))
(indent-region (point-min) (point-max)))
Would indent the lines, but instead of actually changing the buffer, it would put a display property over the leading white spaces, where the text in the property corresponds to the correct indentation.
The rest of the time, (when non-persistent-formatting is nil), then it removes the display property on the leading white spaces, and modifies the line the normal way.
In this way, I can actually understand
the file without causing chaos in the source repository.
I think that would be a killer feature. That's my .02$.