Having played this pretty extensively, I managed to notice that Mario himself feels off compared to all the other characters, who are otherwise pretty much perfect (Simon of course has been buffed for obvious reasons). Mario seems to run a little too slow and skid a little too much.
The creator said there may be sequels. All I'd really want him to do is balance the characters a bit more (Bill is pretty broken), maybe add another one (Kirby? Arthur from Ghosts n Goblins?) and possibly have more than just Mario levels.
Hell, maybe the next came in the series could be based on a different game entirely. Legend of Zelda Crossover? Mega Man Crossover? Endless possibilities here...