I'm just going to paste what I previously said.
1. Maybe even a household theme and were shrunken in size and can interact with things like hangers or lamps.
2. I'd actually like a Yoshi world. It'd basically be filled with multi-colored Koopas that give you power-ups, Yoshi eggs everywhere, and the landscape is modeled like in Super Mario World with green mountains with eyes and rising hills.
3. You know, how about a world based entirely on a horror-theme? You'd basically go through graveyards, spooky woods, tunnels, mansions, and even dungeons. We could even meet the ghosts from Luigi's Mansion again.
4. How about an underwater cove world? It'd have you going underground and underwater in one world and have tons of sea life, boulders, and even sunken ships like the one in SMW.
5. Or maybe a western type canyon, full of whirlwinds, tumbleweeds, stampedes, mountains, gouges, earthquakes, and strong winds. it'd be pretty awesome to ride Yoshi there as you'd look like a cowboy.
6. How about a vehicle world? Basically , the landscape would be a road, with tons of vehicles coming in and out you can ride and avoid. There are cars, trucks, and motorcycles on land, boats and submarines underwater, and planes and jets in the sky.
7. I want a prehistoric world with dinosaurs, volcanoes, plains, caves, and rocky terrain. There could even be meteors in one level referencing the how the dinosaurs got extinct.
Mario4Ever said:
Miyamoto's mind (Mario has to travel across and combat elements from Miyamoto's discarded ideas).
Epic Mario.