For me it will either of these, And I hope and bet that R* will pick one of em.
• Chicago (Present day) • Detroit (Present day) The two cities could combine. • Las Vegas in the 70's or 80's when the Chicago Outfit and various other Mafia families owned the Casino's, not as much like it today.
• Vice City, maybe V and VI and IV will be all connected. Vice could be about the diamonds. Maybe a 80's remake but following the all too violent Colombian cartels at the time.
• San Francisco in the 70's and 80's, following the hippie and smoking weed era, a long with the Triads and other East Asian gangs.
t's time for R* to invent something new! a city can have the feeling and "vibe" of a real one, but must create a lot of fictional landmarks (while GTA IV is the opposite: a "realistic" city with few unique landmarks and houses). Liberty city in GTA III was a good mix of reality and invention, so why not doing something similar (and maybe with section from different cities, not copying from one city only)