A sequel to the original action role-playing game Marvel Ultimate Alliance was inevitable, since it sold oodles of copies across all platforms and was even part of an Xbox 360 bundle last year. Back in February 2008 it was announced that the sequel, was indeed, in the works and it received an official subtitle a couple of weeks ago: Fusion.
The "Fusion" subtitle relates to the unique way the superheroes will be able to combine their powers. This kind of thing has been done previously to a more limited extent, like Colossus' and Hulk's ability to throw Wolverine like a projectile. For Fusion it appears this kind of combination will be the norm.