Rockstar San Diego appears to be hiring for their as-of-yet-unannounced next project, which they're billing as another "open world" game.
Over the course of the last month, the developer has posted six new job listings to Gamasutra, seeking various programmers and artists to work on the studio's latest project. Three of the postings make special note of the fact that the new project will features "open-world" gameplay.
Rockstar San Diego's past projects include the Midnight Club: Los Angeles and Red Dead Redemption. Both of those franchises fit the open world bill quite nicely, so we might well be seeing a sequel to one or both of those games in the near future, though the studio could also be working on a brand new IP.
Of course, we've got nothing but speculation to go off at this point, and there's nothing concrete to indicate that Rockstar San Diego is even working on its own game at the moment, as Rockstar frequently employs multiple studios to work on its high profile titles.