Something I'm kinda working on in my spare time (which I currently have none) is the Bowser Suit. It's done in 8-bit style, as if it were actually used in a real Mario game. The sprite below is the on-map icon, as the Bowser Suit cannot be found in a level, only purchased from a Toad House (or whoever sells Bowser Suits).
The Bowser Suit is basically the most amazing thing ever, as it combines all the best powerups from Mario 3 -- the Hammer Bro. Suit, Fire Flower, and Kuribo Shoe -- and gives you the ability to ground pound, which shakes the earth and is like using a POW block.