Mario Galaxy - One (or several) two dimensional planets that characters fight on. If there are several, then they can have different levels of gravity, which will affect how easy it is to knock someone up.
i can just picture a platform, pretty decent in size, taking up most of the bottom part of the screen as one surface of a planet. then other smaller planets floatind around that act like platforms but with their own gravity, make it possible to fight upside down!
you think you're sending your opponent flying off the stage but the gravity of one of the planets/platforms pulls him in instead, that'd be awesome.
as for the old school bowser levels.. i could totally see a slow paced scrolling level with sparkys flying out of lava pits, fire chains spinning from floors and cielings, fireballs coming out of bowser statues. even the "high/middle/low" puzzles can be thrown in.. taking the "wrong" passage could deal a small amount of health, or taking the right one could give you back some health perhaps?
and speaking of olschool mario stages.. we need more pipes. pipes in the first MK stage from the original smash were cool, and i really missed them (and the pirahnna plants) in melee. they need to return.
also, a fun new item should be one that turns all characters on screen into their 8-bit forms. granted not all characters have 8-bit sprite forms but it would be nice to see what the artist can invision someone like fox or falcon like. it would just be an item for fun that would have no negative or possitive effects. if thats too lame, heres a pro/con it could have:
-turn everyone else 8-bit while the player who used it remains intact. the unnaffected player would be larger and faster than the sprite versions.
or
- all characters turn 8-bit and special moves (B moves) are turned off until the affect wheres off.