(Not to get ahead before GTA IV). i think they should move away from past GTA cities and start some where else.
If Asia, then Hong Kong would be a good pick. Its near the coastline and populated. Japan is great to extremly populated and very active. There would be problems against it though. Like the langauge they speak and so on.
Europe is a good choice. And I will stick with London for so many reasons. The under-ground crime story sounds good (Total-Junk).
Going back to me saying they should start with somewhere new if not. Then Vice City is a must. A fresh bigger newly designed VC would be a tremendous job. Nightlife, and a whole drug dealing bussiness would be great. Add in cars,palm trees, women, and money. Great. With the new rage system perfect. I also think that Rockstar should work on their airports aswell.