5e has done so well, like 3e when it came out. Not sure what they would want to change like making 4e seemed to change a lot and people we not sure it was the same 'feel'. So to change the edition enough to buy all new everything and still be D&D night be hard.
What to dump and what to keep and what to change. I might dump subclasses and add something like 4e with gaining a choice of powers at certain levels. This might allow each class to not be exactly the same at each level. People like feats and it allows some customization, but I think there is some work that can be dome there. Spells can be completely changed to me with losing half of them or limiting them to only some classes.
There might be things not touchable and are too sacred. The ability scores, the HP, weapons, armor, that kind of thing.