Probably another fallout game or something, but if it were up to me, they would devote 100% of their time to TES. It is the most important thing to me.
Its bad to put all your eggs in one basket. Mind you...TES is probably one of the best baskets in history...but those eggs are still easily broken.
Not to mention I don't mind them gaining some experience in making another game. You could see it from Oblivion to Fallout 3. Look at the transition from a Daedroth to a Death Claw! They go from wimpy to something worth fearing. Now I can see some of the things they took from gouls in Fallout 3 and infected the zombies of TES! I can't wait!