If GTA really hits off and it retains its popularity long term, obviously there are going to be so many things for Rockstar to add as DLC etc to keep things fresh.
One idea I had, which is a bit 'sci-fiey' and perhaps moves away from your traditional GTA style but nevertheless could be very interesting, is introducing space travel and planets etc.
Think about it, we could purchase a rocket (would have to be very expensive) and fly the rocket up towards different planets which we could inhabit etc.
Perhaps Rockstar could do something with the content creator so that we can create our planets environment etc.
I know it sounds a little bit different, but it would be fun for sure.
The could add different guns like lasers and maybe even light sabers etc