Grand Theft Auto V, Grand Theft Auto Shm-V. Rockstar's latest can wait, as modders are busily compressing the previous two games into one glorious, sort of broken whole. GTA IV: San Andreas is a total conversion mod for Grand Theft Auto IV, and ports the giant San Andreas map of Grand Theft Auto: San Andreas into its RAGE Engine powered sequel. It's just entered its third beta, surprisingly enough, which adds: "multiplayer, races, gang wars, improved Setup and Launcher, improved graphics, map, vehicles, sounds, peds, shaders, collision materials, timecycle and plenty of other improved stuff to check out." It's still early days for GTA IV: San Andreas, but the below video show strong progress.
I just tried it. It crashed immediately. Lovely. Godspeed, you frantic, asset-porting madmen.