For instance, take GTA IV’s solid if unspectacular online modes and add them to a PvP environment, add a faction/guild mechanic, travel to multiple cities and areas around both America and perhaps the world, modify the quests to incorporate more than one player character and maintain the great satirical nature of the previous games.
With those additions and ideas from the creative minds of Rockstars designers, this could be the megaton bomb of massively multiplayer games (with the obvious exception of WoW) in the future.
A sticking point to its success would be the longevity of the gameplay and levelling system. As GTA doesn’t have a specific advancement system, it seems hard to imagine Rockstar creating a legitimate advancement map, at the first attempt no less.
However, if that could be remedied with external help from MMO veterans (Blizzard, Cryptic etc), then a solid foundation for weathered players would help greatly in cultivating a strong community and experience.