How about a world that takes place in a town occupied by bowser’s forces? Platforming could take place on streets, on rooftops, and inside buildings. Maybe you sneak through a sewer system or over a walled section of the town. Maybe you can rescue citizens and they award you with powerups. Finishing a level should liberate that part of the town.
One enemy I found interesting in Mario World was the magikoopa. Where exactly did these guys come from and where did they train? I’m picturing the magikoopa world as something similar to the Arcane Sanctuary of Diablo 2, where the place feels not quite natural and governed by ancient magic. What if you found a magikoopa rod and gained the ability to turn blocks into monsters? I’d love to create my own personal army even if they were just disposable flunkies. There are limitless gameplay possibilities with this type of world.
The magi-koopa question is great. Where did they come from? Is there a land full of magic? We know the kings of the Mushroom Worlds have wands and that the Koopa Kids stole those wands and turned them into animals in Super Mario Brothers 3. Where did the wands come from? Where does the magic come from?
Would it be a Magic Land or something like a theme from Arabian Nights with genies and lamps and all? The force is strong in this. Anyway, it would be more interesting than another romp through Desert World or Water World.