I've noticed a few people mention that they think fusion would be Awesome, but there is a few problems with that idea.
Firs of all, If it involves two players (which it should), one player would have to just watch while the other player actually plays the new fused char, unless they somehow finagled a way for both players to play which would probably make it useless unless both players had some amazing communication skills.
But what if it didn't involve two players? Then what's the point? The only reason you would WANT a fusion ability is to fuse with a friend or some such, so making a fusion with some random NPC would make it a pointless and boring transformation.
What I'm trying to say is Fusion would never work in an MMO.