There are already threads about which city / time age ppl want GTA 4 to be in, however the replies there are just shoutouts. To write a proposal here you should think through the idea, area and motivate why it would be especially cool to have GTA 4 there. (Not like: GTA4 should be in Seattle bcus im born there etc.)
An example for how a suggestion about GTA:SA could have looked:
GTA:SA is set in western USA, with LA, SF and LV as cities. The time period is early 90:s with LA as a low-tech city almost taken over by criminal, battling gangs. With SF as a high-tech city and LV as the luxyary, low-paced place all three cities will be unique. The story is about a former "gangster" returning to his old "homies" and "gangsta rappers" in LA to retake the city.
And then to my idea:
GTA:4 could be set in a map referenced to three areas in the world. Firstly the largest part is in China with Peking or Shanghai as major city. Here the player can start to build his reputation, and also have a base for properties (manufacturing stuff, growing drugs etc).
The second area could then be Japan and Tokyo inspired. Here the player can sell loads of stuff, cet incorporated with the Japanese mafia and just earn a hell of a lot of money.
In the third area the player could come either to Hawaii (semester place, beaches etc), or to North Corea (extremely militarized and dangerous zones). Or why not both?
The time could be set around the fall of the Soviet Union (1992) to create a very unstable atmosphere especially in China and North Korea.
I also would like to add Power plants and vehicle transport trucks and ferrys, among other things.