Give or take 5 years. I loved the feel of GTA III & SA, Vice City was great, because of the stuff you could do that you couldnt in GTA III. For me, GTA was all about the gangs. They were great in GTA III, the yardies, diablos, mafia, cartel, triads etc. VC gave you a more involved feeling when you took over diaz's drew. However the only other notable ones were the cubans and hatians, and I think r* went backwards when they made the game based in the 70's. Yes, crime and drugs (a lot of which were just being recognised) were rife in that era, but only because police technology wasnt as advanced as it is today. The same shit still happens today, its just hidden from view, and not pushed into the public eye as it was then.
If you want mafia style 40's and 50's, go play the godfather. Its a good game.
I wouldnt like to see it set in the future... The main reason I like GTA is because its realistic, doing a drive-by on the top floor of a 50 storey building with a crew of robotic lowlifes in a flying mercedes doesnt really appeal to me.