I'm playing my millionth game right now (rounded up) on expert, and I thought of one feature that would be nice to have.
I never play by trial-and-error, I only click on paths when I'm sure of it, and I end up clicking the "Clear mistakes" button after every single path I click, to make sure I didn't miscount something, which is getting a bit annoying.
It would be nice to have some sort of option to automatically mark mistakes as they happen, like a small "X" on the path, or a different color or something.