There will never be a "GTA: London" because the gta games will no longer be based on one city (the major games, not time fillers like LCS and VCS). SA had 3 cities, and the PS3 will have loads more memory, so I can expect 5, maybe 6 cities, maybe even more if they use less countryside between the cities, or if you travel the cities by plane. Of course, if they do "international", London will be one of the cities (there will never be a whole game based on London). Also if it is international, one of the cities will in america, because international doesn't mean "not in america" it means "the whole world" and america is a big part of the world.
The american city will probably be a new one, trying to portray all the major cities in america in one big city. If the main character is american, it will start in this city.
Paris will most likely be another city later in the game, not very big, like the size of San Fierro. If they use fictional names, it will probably play with the french accented pronounciation of Paris (Pah-ree) and be named something like Pahri or Paeree.
A Tokyo theme is almost as obvious as London, as they are a must in an international car game (have you seen the cars in japan?). The Yakuza will make a return if Tokyo is featured, as the Yakuza are the Japanese Mafia.
And of course, we need waving palm trees somewhere. What a better place to put them than Brazil. Brazil has alot of crime in street gangs (like a brazillian GSF), and is very similar to Miami/Vice city in that they are almost identical (in RL I have major problems telling the difference between the two).
So thats 5 cities I thought of, 3 of which are obvious. They could end it with this, but if they do a 6th city, I dont have any clues onto what it will be based on.
So this is what I expect
Urban American City, skyscrapers, mix of major cities (new)
London
Paris
Tokyo
Waving Palm Tree city (Brazil, Bahamas)
The last is up for speculation
Of course, they will all fictional names that are similar to the city they portray.
One thing's for sure, GTA IV will kill all the other GTA games, to the point that the other games will be considered "classics". Also, this episodic content I hear of mentions new areas, and you might see yourself downloading LC, VC, and SA onto your game, which will really kill the other GTA games.