Technically we hadn't seen 3D Mario in a 2D style before 3D land and world, the difference is there wasn't a more a more cut down version of Galaxy available handhelds before it released etc otherwise you wouldn't really be able call that ambitious either.
3D mario in a 2D style is playing it hella safe. I know why they did it, but I wanted something else.
Yes, it would have been challenging to design (especially the camera) for multiplayer, which I also wanted in a 3D mario.
It's only a "world" as a throwback to super mario world.