ok, just a sec, i was just reading this cool article about a game for the wii where you play as a scorpion and a tarantuala in the sonoran desert.
i'll summarize it:
Stage size: do you want an offense heavy bout with lots of kos, or would you prefer a grind it out defensive battle where heavily damaged characters can stay in the fray? the bigger the stage, the more room characters will have to recover from being launched. also, consider what types of characters your stage will favor. small spaces benefit slow, heavy characters, quicker characters thrive with room to hit and run, and distance to attack with projectiles.
launch areas: a wide open area will encourage a large ko count, but a level with walls, ceilings, or other obstacles can keep damaged characters in play much longer, requiring more cunning offensive strategies.
controlling spawn points: you can place spikes on top of any high surfaces you dont want characters spawning on. putting spikes on top of a box shaped arena will put them in the box instead of on it. also allow characters to reenter the combat zone after falling: they always reaapear at the top center of the screen.
gimmick stages, like ones made only of ladders and dropping blocks, get old fast. a little amount of special parts goes a long way.
sketch out ideas with smaller blocks and pieces, then look for ways to replace groups of similar parts with a single larger one of the same type.