All the cities mentioned are horrible ideas. NYC has been done twice with 3 and 4, Miami with vice city and compton with san andreas. I think Hollywood or Vegas would be a perfect setting for the next GTA. I cant believe none of these two were mentioned. Vegas dominates Atlantic City in every aspect. As for Hollywood, need I say more. If they did Miami then Hollywood would be great. (Btw im in NYC for those that think im biased).
If you played gta 3 you'd notice it's not nyc, it's just a city with that type of eastern flavor, GTAIV is the first to fully replicate NY. Also, in san andreas there was an island with vegas in it, remember all that mountain and desert... not good game material, and remember the vinewood sign too?