I don't see the value of the open world thing really. Seems like a lot of needless running between locations for Mario. Best just have worlds a la Mario 64 but make some of them a bit bigger and more complicated to enable even more exploration. But then this still allows traditional missions and the changing of the worlds for different missions if so desired.

For the record, Mario 64 wasn't open world at all.

Yup, SM64-2 was supposed to come out for the 64DD, but that thing was a damn failure and thus 64-2 got cancelled. What's noteworthy is that 64-2 was supposed to be the first 3D Mario game with multiplayer, meaning that Luigi would make an apparence!

EDIT: My bad.. Mario 64 also had multiplayer originally, but it got scrapped..

Well if there was gonna be a a more open 3D Mario in the vein of 64 with multiplayer, the Wii U would be the perfect system for it. Splitting the camera between the tv screen and the game pad would certainly fix the issues one would encounter in a free rein 3D environment without having to resort to split screen.

I love how creative you are with your definition, but every Mario since world is actually open world.

What's more likely, Mario 64 2/Mario Sunshine 2 or Mario Galaxy3?

I'm thinking that with Nintendo's attraction to couch co-op, the idea of Mario 64 2's multiplayer could be revisited, especially with the option to each have your own screen.