I think the area of the game will be very important. Whether the map will be the same size as GTAIV or not, I'll predict London has the best chance due to the volume it has w/ European gamers and the fact it is a bigger crime city with subway bombings and assassinations.
But if the map could be huge, and I mean huge, then combining aspects of the middle east (Dubai), the central Asia (Shanghai), and eastern Asia (Tokyo) can make it a very large game.
But I hope Dubai is a consideration, the city has some of the greatest skyscrapers, middle east setting is recognizable, and the crime is massive.
Above I'm a proud Gran Turismo fan, not a Sony fanboy, and now a proud 360 owner, but sharing xbox live accts ATM
End of 2009 Predictions (made Jan 1, 2009): My predictions were pretty accurate, the 360 was over and the Wii was just under.
Wii: 65 mil (yea I'm crazy) 360: 35 mil (its getting there) PS3: 30 mil (the slim better do well)
End of 2010 Predictions (made Jan 7, 2010):
Wii: 81 mil 360: 47 mil PS3: 45 mil
