I gotta encourage people go for a castle stage from YI!
It would be a great change-up from how YI stages are.
Ideas for my stages:
Mario:
Good Egg Galaxy - Basically, a gravity system. It all depends on the planet - the first planet is large, and as such doesn't allow you to go all around it, but you can jump onto a planet orbiting it, and fight upside down. The players automatically all get sent to the next planet at a predetermined time, which may be completely different.
Doom Ship - Scrolling Doom Ship stage. Was kind of surprised it wasn't in SSBB, since they did SMB with SSB and SMB2 with SSBM.
Mushroomy Kingdom 1-3/1-4 - Scrolling stage of 1-3 and 1-4. Pretty self-explanatory.
Light World/Dark World - Basically Hyrule Castle and Pyramid of Power. If a portal appears, any player entering it changes the stage around. Enemies appear on the stage, and they change as well.
Pokémon Field - Basically a Saffron replacement - it would be surrounded by a city and a cave, with a patch of grass and a pond in the middle. Pokémon could come from any which way, including from a Trainer.
Mt. Dedede - Kind of like Little Mac's arena, but overtime, the arena would raise to a windy roof, and at times, Dark Matter will be waiting.
YI Dungeon - Would have various elements of a YI dungeon - some enemies, Ferris wheel platform, spikes around that do lots of damage, and a turning mace.
New Pork City: Good Version - Limo as platform regularly, full of platforms (model it more like the real New Pork City), Yokuba!!, sewers.
Magicant - Ness'. Flying Men would be like built-in ATs that would fight alongside the person who finds it.
Tanetane Island - Would basically be really, really scary. Like Touch Fuzzy, Get Dizzy without feeling gimmicky. Eerie Smiles would attack the players, one of the platforms would be a "hot spring", players could heal when a purple mushroom appeared, etc.
Classic Ice Climber - Would not be a climbing stage, but would have classic IC visuals.
Wars World - Tanks, infantries, and bombers would pelt the stage with ammunition. Various terrains would be more or less protective. Player could hold down R to change it from green, grassy area to ruined area as in DoR.
Smashville - Would be ground-based, and villagers would walk around, chat with each other, and give you items sometimes. Trees could yield items or bees.
Kingdom of Zeal - Would be huge, floating platforms, Epoch would fly around it as a platform, and sometimes, Lavos would shoot his heaven beams and send it into the ocean.
Rhythm Heaven Remix - Would contain the contents of five rhythm stages from Rhythm Heaven, including the Karate Man stage. Would switch between them, and hazards would be presented in such a way as they make sense, like the Karate Man punching objects.