I personally think there will be more diversity.
If you notice Drake, Jay, Kanye, Wayne, Nicki etc (even if they are all mainstream) have their own sound going on.
Kanye will be the one in the forefront with more abstract, experimental music. Drake will probably stick to his ambient, chick stuff. Jay, i honestly don't know where Jay is going. It's hard to tell since he's pretty much done it all. Jay drops one super commercial album but then manages to bring it back with the next (black album, KG, American Gangster, BP3) If the pattern is correct, he'll have a stronger hiphop based album sprinkled with some commercial tracks here and there.
The pop stuff will continue merging with dubstep. Gaga will continue infusing old school with new school, and will probably mix more classical sounding stuff into her pop music.
Madonna will be back.
I hope 50 finds a way to reinvent himself but i see him more involve in business. 50 needs to collaborate with artists outside of his circle.
Rick Ross and MMG will continue dropping decent music.