Another idea I had was a Star Wars edition of Mario Kart.
Mario -> Luke
Luigi -> Han (or Obi)
Peach -> Leia
Yoshi -> Yoda
Koopa -> Storm trooper
Toad -> Ewok (or Yoda, in which case Yoshi would be Palpatine)
Bowser -> Vader
DonkeyKong -> Chewie
Each one sitting in a spaceship rather than a gokart, Vader in a mini star destroyer, perhaps. All the graphics in each level would be changed for something more appropriate, not to mention the weapons. This should all be doable with a good rom hack. The music and sounds should be changed too. There wouldn't be much point to a game like this other than as a joke but it sure would be fun to look at for a few minutes.