I would love a more cartoon like style. Music that is reminiscent of Super Mario World. Keep the 4 player chaos though, that is so much fun!
I've thought it would be interesting to try a cartoon style again, but to me I also want a return to the feel of Super Mario World. The journey and the world just felt so much bigger in World than any other game. Every other game just felt like a collection of levels organized into 8 groups called "worlds", including 3D Land and 3D World. With Super Mario World, there was so much going on and it felt much more memorable. Remember Yoshi's house and the excitement of finding him for the first time, and how his friends were missing, and you wondered what they were like? How exciting it was to find the different colored Yoshis and see their powers? Remember when you first tried the yellow switch, and when you saw the red outlines you immediately wondered where the next switch was and what new possibilities it would unlock? Remember wondering what each new Koopa Kid would be like? Remember beating them and the narrator telling you what the next chapter of the journey would be like? Remember what it felt like to find a key and take it to a secret exit, and unlock a hidden path? Remember trying to break out of the Forest of Illusion? Remember your first ghost house? Remember when you first discovered Star World? Or when you got to the center of Star World and went to the secret levels? Did you ever get to the bizarro world after the secret levels where all the sprites are changed? And as far as I'm concerned no Bowser fight, at least not in the 2D platformer games, was as climactic as SMW's.
No 2D Mario was as memorable as memorable as World. Each area felt like it meant something to the journey rather than just being another collection of levels with another theme that usually had already been done in another Mario game. It had levels worth exploring that had more than just 3 hidden stars/big coins/etc. slightly off the beaten path, sometimes having completely branching paths that led to secret levels that led to secret worlds that led to more secret levels. Other Mario games maybe had a secret exit or two that best case scenario unlocked a Toad house with a bonus item or a slightly different path that came back to the original or let you skip a world or something. Super Mario World was the only one that had all its levels on one sprawling, interconnected map that felt like you were traversing a world rather than dotted lines connecting a string of levels. The bosses felt different every time and there were even secret bosses. Super Mario World was 2D Mario at its most interesting, by far.