Rockstar Games has already confirmed that Grand Theft Auto VI is on the way. The company has already made games based in Liberty City (GTA III and GTA IV) and has already done Vice City and San Andreas on a number of occasions (GTA San Andreas and GTA V). Liberty City is based around New York while Vice City stands in for Miami and San Andreas for Los Angeles.
With these cities done to death, here is a list of other cities that Rockstar could take a look at.
London & the European Union
The series has already taken place in London in the past and how interesting would it be to see a new 3-D entry in the franchise featuring the hero taking on missions in subversions of historic landmarks. How about flying a helicopter to an alternate version of Big Ben? How about raiding a different version of Buckingham Palace? Maybe instead of focusing on gang bangers and drug dealers, the series could take a more political turn in London?
Since GTA VI will probably be on a bigger console, why not expand the map and let players jump between the European countries in a big political journey? This would lend variety to the different maps and give players a chance to in engage in a diverse range of cultural activities and ideas.
Colombia
Set the game in the 1980s when Pablo Escobar was at his height and you have a rich potential, especially considering the franchise. Add in the Spanish culture and language and Rockstar can really play with a number of different qualities that it has never seen before. Imagine traversing mountains and taking on armies of guerilla fighters. Imagine doing missions in which the character might have to take down a major cartel boss. Imagine going through small towns and customizing your character with a diverse range of different clothing and objects.
Mexico
Set the game in modern day where the drug gangs are at their height and in constant battle; this would suit the franchise well, especially in light of GTA San Andreas. Why not do something relating to the illegal immigration problem? Rockstar is known for its biting satire; why not throw a few bombs at Mexico's telenovela culture in the new game?
Rio Di Janeiro or Sao Paulo
Brazil also seems ready made for this kind of game. Rockstar could play around with the poverty and favelas. You could play around with the soccer culture; imagine side missions related to the sport or even, dare I say, the euphoria over the World Cup and the protests that are likely to dominate it?
Other major locales
Bollywood is in full swing and could give Rockstar the material to not only satirize but also truly develop. Japan would add a new flavor to the proceedings, especially with its rich artistic and technological culture.