Yeah, Simon would've been awesome. His Vampire killer whip would probably be both his standard attack and his Up-B recovery move.
SUBSPACE EMMISARRY BOSS:
Dracula
Mario would totally own Dracula...
I just though of a stage for him...
GRAND HALL (From Castlevania)
A long hallway with a 2 breakable platforms in the center and a broken, bottomless pit in the center of the stage. On the end sides, there would be a slightly elevated balcony area. There would be a large, ornate glass window in the back of the stage. The stage would continue to switch from day to night. In the daytime, light streams from the window and no monsters appear. At night, however, monsters from the Castlevania series move across the stage and strike the players. Also, Dracula or bats sometimes can be seen flying outside at night in front of the large moon.