Maybe the combined map could be for a GTA MMO..only, if I were Rockstar, I'd make the cities of GTA in massive scale and then just allow players to fly between them. And each city would be a different set of servers. Heck, each city could even have a different set of cars....although, all of the cities would have similar exotic retailers.
But...that's the only way I see having all five cities together: an GTA MMO...which would be sweet.
EDIT: Apparently, that's what Rockstar is pushing the series towards:
And adding a massive map and a character already well establishing in the in-game community are two more components of an MMO. (For GTA, having a character established in one of the cities and expanding to others makes more since for a GTA MMO).
So...GTA 6 or 7 will probably appear as an MMO. And who knows, GTA 5 could have more multiplayer support...maybe some co-op missions or something. It would be interesting to see what Rockstar would do with classes and leveling, though. Although, previous GTA games have done a more...passive leveling system where by the end of the game, certain groups shoot you on site and others just ignore you or help you out. Perhaps with a GTA MMO will see the ability to become a cop. Think something like APB, only a million times better. (A GTA MMO might be the first time players can play as a woman as the protagonist. So...more like a less crazy version of Saints Row.)