I dunno. Mario 3D World is probably the most pure 3D Mario game.
It's a Mario game with 3D platforming.
Whereas the previous Mario 3D games were more like 3D platformers that just happened to have Mario in them.
Maybe Nintendo should just make a new franchise character and make a 3D platformer around them. I do love the Mario 64 formula, but Sunshine was really that formula pushed too far. It didn't feel like a Mario game anymore, it just felt like some generic 3D platformer that had Mario pasted into it.