Single Status Update
One thing I think is a shame about NSMBU is how Bowser basically takes over Peach's Castle and just throws Mario and his friends is away, but it never amounts to much because he basically throws them from one grassy field to another one and by the end of the game, the castle is just another lava world/Bowser Castle, but with Peach's place as a backdrop.
I think it would've been a pretty neat twist if you started out in a pretty easy Lava world and had to work your way back home, with a tough, endgame-worthy, but still happy and harmless looking Grassy final world. Put the concept on its head a little. Maybe make the desert the second to last level and finally make a freaking DESERT the tough environment we know it as instead of early game fodder. Things like that.
Of course, wholly unique environments would be even more of a plus, but if we're gonna stick to the typical tropes, this would've been a pretty fun way to at least shake it up somewhat.