Its going to be probably either
A) Motion controls, as much as "core" gamers like to complain about them they aren't going anywhere fast
B) FPS. Its quite simply the easiest of all genres to make into a large scale competitive game. People are always going to want to compete with each other online so shooters aren't going anywhere.
C) MMO. I have a feeling next-gen both Sony and MS are going to pursue creating large scale MMO's for their consoles. They've seen the success of WOW and there is nothing remotly similar on home consoles, trust me they see how much a company can make off a good subscription MMO and they are going to be interested in seeing similar sales.
It really depends on how it all plays out but I think all 3 of those will be contenders for biggest genre.







