Super Mario Fusion might be the best Mario to date.
Created by a fan, it will feature guest characters, widely varied worlds, and a completely new weapon system in addition to the standard Mario weapons (shell, fireball, etc.).
To quote the creator: “Think of this game as Mushroom Kingdom Hearts of sorts.”
Planed worlds include Sonic, Megaman, Castlevania, Halo, SHMUPS, Nintendo, and more, and each one will be unique.
For an idea of the end result, check out this video of the Halo level: