im looking to make a 3 way mashup, the concept idea is interesting enough anyways, we've got Castlevania, Ghosts N Goblins, and some Battletoads, the idea is still a linar beat-em up kinda game, the idea also came to me after playing Mushroom Kingdom Fusion, which I think uses the fusion game engine, but it was a fairly solid platformer,
the game reguardless if your Simon, Trevor, Arthur, Firebrand, or one of the Toads, would progress in a fashion like
eg: Stage 1: Is say Battletoads NES stage 1, then Castlevania 1's stage 1, followed by GnG stage 1, it would give a bit of a break in terms of difficulty, but build it up each time in a 3 part act so to speak.