They could always try setting the story in a foreign locale, as already mentioned, going back to ideas from the first two games, or even revisiting cities they've done recently. I'm sure they would love to remake and upgrade GTA III at some point, or expand on Las Venturas.
England. Russia. South America. Mexico. Hong Kong, Taiwan. Japan. Although gathering soundtracks for many of those... yeah, that's the problem. The best chance of any of those would probably be Japan or England.