The folks at Rockstar North are infamously secretive, to the point where some game journalists are forced to sift through LinkedIn descriptions to find out anything about what the Scottish devs are up to.
An ex-employee at Rockstar Leeds had this to say about his time at the company:
This, combined with another Art Assistant's profile who claims to have worked on 2D and 3D art, suggests that Rockstar may be working on a handheld title of some sort. The "unannounced DS title" may still be in the works, and the part about a "complex hand to hand combat system" sounds delicious too. For now, however, let's just stay calm and wait patiently for the news to come to us.
Source: SystemLink Nintendo