I was actually just thinking of this! I have only ever posted in the GTA V section of the forums. I came here to see if there was any talk on Vice City for VI.
The III era had Liberty City, Vice City & San Andreas. The IV era games were based in Liberty City and the V era is based in San Andreas (I wonder if we will see San Fierro/Las Venturas in the DLC?). So maybe VI will be set in Vice City? I would actually like that. VII will have to be something completely new.