Since it's been a long time since we have seen Vice City(Miami) in a GTA game, I think the next Installment is going to be back in "Miami." I don't think that Rockstar will use a different location other then Miami, NY and LA since those are the only major popular places that have large bodies of water surrounding them in the U.S.
HERE IS WHERE THINGS GET CRAZY
If i'm not mistaking, the first ever GTA game ever included all the three major locations which are, Los Angeles, New York and Miami. Rockstar could do this in the next installment of GTA.
Maybe we could go to the airport, get on a plane and time between those 3 locations, after all the next major installment will be on next-gen.