I think in the next GTA, Rockstar will allow some form of DLC to be put in by the players themselves. It's like how Halo allowed players to make their own maps in Forge. Rockstar could allow players to use a tool that allows them to make their own cars, structures, etc. It's like mods, except Rockstar actually sanctions it and it can be used on the 360, PS3, PC or whatever console they release the next GTA on.