According to this interview IGN had with Shuhei Yoshida, all of Sony's first party studios are making PS4 games. They have twelve studios, and of those twelve only four have announced games: Sucker Punch is doing inFamous: Second Son, Evolution is doing DriveClub, Guerrilla is doing Killzone: Shadow Fall and Japan Studio is doing Knack (and maybe also possibly The Last Guardian?).
This leaves us with the following eight studios to speculate:
I think it is pretty obvious what three of these studios are working on: Polyphony is probably doing Gran Turismo 6, London is probably doing a new Move game or maybe a SingStar game, and San Diego is probably working on the next MLB game. As for the rest of them, I personally want Media Molecule to work on a new 3-D platformer but with the Move presentation they gave I am not hopeful. I also would love to see Naughty Dog create a new IP for the new generation and take us through a new trilogy. I would want to see Bend work on a new Pursuit Force game because I think that franchise still has some juice left in it and with BigBig gone they would be the next best people to make it. I would want to see a new Medievil game from Cambridge. And for Santa Monica, I would love for them to focus their efforts on either a new Twisted Metal or PlayStation All-Stars 2 before doing God of War 4 and develop them completely instead of making them side games given to B-teams.