First and foremost.. I'd love to see a true return to the fundementals of classic Mario, simlar to how Prime was largely a return to form of the spirit of the original and Super Metroid games. I want fundemental, fast-paced platforming, and ditching the star fetch quest, while tightening up the controls and overall speed of the game.

In terms of the gameplay, I'd like to see some new themes, and I don't mean dry, uninspired, and repetitive concepts like galaxies, but rather, a brand new environment much like dinosaur world. At the very least, a reimagining and/or more fully fleshed out version of the mushroom kingdom would also be very interesting. I think I'd also like to see an increase in difficulty; removing over-powered powerups and/or life bars and returning to the one or two hit system would be a start.

i really don't care as long as it's not Galaxy 3. that would be ridiculous imo, to keep that going.

Something like Mario 64 but even more open world.
I don want Galaxy 3.

Spam replaced by the One True King

An open world structure just like Mario 64 and Mario Sunshine! The Galaxy games and 3D World are amazing, but I am getting tired of the lack of non-linear levels!

Right on! It can only be Super Mario Universe. And not Universe as an expansion of Galaxy, but an ode to the whole mario franchise/universe.

I'll build my own 3D Mario game with blackjack and hookers... In fact forget the 3D Mario game

I would like to see Nintendo make an entirely different 3D Mario game than the previous ones. I want to see a Super Mario Adventures game where it is an adventure game merged with platforming elements we've come to associate with Mario games but more adventure than platforming. This will help separate itself from the previous platforming Mario franchises we've seen in the past.

The adventure game will have no time limits and have a particular theme to it. We've had the space theme in the Galaxy series and we've had the tropical theme in Sunshine so its time for a new theme. For some reason I envision a theme based on Arabian Nights or Ancient Egypt - how awesome would that be.

I think I might want to have Mario in it. Maybe.