re: Was there any single thing that Hillary Clinton would like to do over?

The whole Hillary experience was so unbelievably flawed that you couldn't fix it with just one change.

The closest thing to that would probably be an attempt to tack toward the middle and salvage the rust belt states like Ohio, Pennsylvania, Michigan, etc. But that would have lowered her turnout of other groups because her coalition in general is hostile toward middle class and lower class white America.