3: Go To Liberty City
Mission: You receive a mystery call on your cell by some guy called Niko. He implores you to get on the soonest plane and go to Liberty City to help him with a drug deal gone bad. If you do it, he can offer some important information about the next heist and offer a new one, the grand Liberty City Bank.
Likelihood: This one isn't very likely, meaning an entire crossover of GTA IV and V. Liberty City would have to be totally revamped with new graphics, all for a few hours of gameplay. Also, from what has been seen in trailers, why would anyone ever want to leave Los Santos for a dreary urban jungle? There is pretty much no way that something like this will happen, at least not in the main game. With self-contained DLC? We know Rockstar puts a lot of effort into their downloadable content, providing 6 to 8 hours of story in both The Lost and Damned and The Ballad of Gay Tony for GTA IV. Rockstar have said that they would like to connect the GTA cities, and have one huge map, but I think that exciting prospect lies solely in next-gen consoles.
Verdict: Very unlikely, but possible DLC?