I really like the aforementioned idea of a Mario Bros Jr. spin on this.
You can play as shell creeper, side stepper, or slip ice. Each one behaves like it normally would and for instance, if you played as the side stepper, you could take two hits before you lose a life. After your first hit you can move faster than before. You see where I'm going with this. Slip ice could turn platforms into ice by pressing Down+Jump and the enemies would all be Marios and Luigis and still fighter fly because nobody wants to play as fighter fly.
Maybe not a great idea, but I like the thought.