I've been playing around with some Mario stage ideas.
Delfino Plaza - home of Mario
Super Mario Adventure (World 1-1) - home of Luigi. It would be an auto-scrolling stage modeled after World 1-1 from Super Mario Bros.
Mario Circuit - home of Peach.
Bowser's Castle - home of Bowser. The stage would have three levels: the bottom level would have lava and bottomless pits with four platforms in between. The middle level would be half platform, half drawbridge with an axe at the end. The top level would be a large flat platform where characters would respawn after being KO'd. Fireballs would be shot from right to left at random intervals.
Battle Stage - home of Geno. It would be inspired by the various Mario RPGs - it would be set on a stage, like Paper Mario: Thousand-Year Door. From time to time, the background would change, accompanied by various effects:
-Standard Background: Draws from Paper Mario and tTYD. Enemies wander onto the stage, or throw items onto it; in addition, the Star Spirits occasionally make an appearance, performing a random effect (recovery, adding or subtracting stocks/time, etc.)
-Bowser's Castle from Super Mario RPG, with Exor in it. In this, Smithy would appear and smash the stage with his hammer, randomly altering the terrain of the stage. During the time after Smithy but before returning to normal, Smithy's Weapon bosses would randomly appear and cause havoc.
-Flipside from Super Paper Mario. Count Bleck would appear and flip the stage from 2-d to 3-d; the characters would still fight in a 2-d plane, but the screen would scroll from top to bottom, bringing in hazards like pits, traps and various enemies from SPM.
-Beanbean Kingdom from Mario and Luigi: Superstar Saga. I haven't played M&L:SS for such a long time that I can't remember much about it, but it would definitely incorporate Cackletta and Fawful somehow. Also, if necessary, it could have elements from Partners in Time, such as Shroobs or some sort of time travel.