But WHY everyone want it to stay on 2D...2,5D is a bullshit marketing label. Everything they have presented are playable on a damn 2D PLAN, so far there is no 3D mode, and it is completly technically possible to propose a Mario 64 3D builder easy to use for casual players.
I think they are more referring to the style as opposed to the dimension. It would be blocky, perhaps featuring panels that you could jump through, and honestly I'm not too sure what else they are hoping for. Mario 64 has been outdone by many, many 3D Mario games and is like the awkward teenager version of subsequent games. Nostalgia is weighing very heavily on that desire, I think. But I am open to those who desire this style to be implemented to elaborate on what about the 64 style they would like to see.
In their defense, I think there's a good bit the 64 version could add. And I would argue nostalgia is all that justifies the original Super Mario Bros. style as well. Mario 3, World, and NSMB all outdo the original. Yet I still think it's fun to have it there. It does have different mechanics. Mostly in the form of simpler mechanics. SM64 would add mechanics, like the dives and long jumps, as well as punching and kicking. I believe it has a backflip somersault you can do by crouching and jumping, and a sideflip you can do by rapidly turning around and jumping during the turnaround. Then there's the caps, and the fact that endpoints wouldn't be a flagpole, but rather touching a star that could be at any point in the level. Galaxy would add all this and more, but that's the thing about these styles, they often serve to be just another layer of complexity to the mechanics with little unique just to them. 64 would have just as much unique to it as World or 3 would. I could see them adding both 64 and Galaxy, or perhaps creating two sets of interchangeable styles, with Bros., 3, World, NSMB being one set and 64, Sunshine, Galaxy, and Odyssey being another. One set with flagpole type endpoints, another set with touch the star type endpoints.