Yeah it won't be any time soon but it will be interesting to see what they come up with. I'd want to see more challenging missions though, it just felt too easy the whole way through.
I think next Summer could be a good bet, didn't they release Episodes a year after GTA4 came out? Although with plenty of focus being put on GTA Online they could hold it off for another year or so.
How are the missions in Online BTW? Do you get to pick from a limited range of missions or do they appear randomly? I haven't properly checked the multiplayer mode yet so I don't know. Are all of them heists you need to do with someone or can you go alone, is there a clear distinction?