Let's screw other places, I really want Rust Belt Cities, especially Detroit and St.Louis. Every timelapse is proper for these cities to be used for GTA. Crack epidemic, heroin epidemic, major car industry and its decline, fierce wars between street gangs, economic crisis, falldown and rise again... These cities have seen them all. Economic situations, crime related clusterf*ck situations, being torn down by drugs... what else a GTA game can need?
For continent, I say "Europe". Europe is maybe a bigger melting pot than the whole America continent, I'm saying it for real. Take London for example. London is always a "shaken down place", but the obvious examples for is that one of the world's higher prostitution rate is in London, once (around 2005) UK was torn down by a ketamine epidemic, which even beat down ecstasy, the drug on the throne of UK criminal for all-time. Let's look at other place in Europe. Eastern Europe is notorious for its wide prostitution rackets... Russia to Albania, Albania to Serbia, Serbia to Greece, Greece to Turkey (Istanbul, more specifically). Also Europe has one of the widest drug distribution lines in the world. Russia and Turkey are two of the biggest bridges from which drugs are brought in and pushed out to be sold for. Also there are such countries as Russia, UK, Italy, Albania where the drug gangs can deserve a "honorable" mention.
I think UK totally deserves to be a GTA setting, but my bigger dream is seeing Mexico as a GTA setting if it's an "outside" setting we're talking about. Huge marijuana farms, brutal executions by the drug cartels, RVs used for producing meth, finding a huge pile of drugs under six feet of ground, illegal gun trade... I'd like to see 'em in a very well touched way.