But you know what would be REALLY nice? ....If they bring back the Koopalings, too. Either as another round of fights, or, even better, they could end up being a part of Bowser's advanced attacks that he could learn over the course of the game!
[tornado fang]. YES.
Power-up items would also be win, like using the Leaf to turn into Raccoon Mario and have a different set of specials and a different attack (like instead of his hammer he'd whack enemies with his tail, and have one special where he flies up into the air and then does a giant stomp on the enemy), or a Hammer Bros. Hammer to turn into Hammer Suit Mario and so on. Hell, they can even add in stuff from Galaxy, like Spring, Freeze, or Bee Mario, where his hammer attack becomes disable in Spring, but jump doubles in strength, Bee shoots out the stinger, and Freeze is essentially an ice-type version of Fire Mario. I'd even go so far to accept only being able to use a certain amount of power ups in certain areas, just as long as they're not one-shot areas (like you need Raccoon/Tanooki/Bee Mario to fly around a bunch of stuff, and Luigi needs to be Freeze, Frog, or Hammer Luigi to walk or swim on water, or act as a shield for Mario).