Well Boston or any other location within the US wouldn't be plausible, that would mean the whole story would revolve around Packie McReary instead of a brand new character. Rockstar is known for bringing fresh and brand new characters to their GTA series so I would think the next GTA would be no different. Besides the chances for a brand new character and story arc are endless if you start off fresh, you could give the player the option to be a good criminal or bad criminal with multiple mission choices which lead to endless outcomes within the story, believe me, this is where their going with the series in terms of missions. The story missions that were most interesting were the ones where you had a choice, TAKE THE DEAL or KILL DIMITRI RASCALOV, mission choices are original to the GTA Series.
Also we must consider the money it takes to make these games and the various restrictions Rockstar had to put up with as of late to do these games. It would be much easier and cost effective to make your first two next gen games based on areas near your various studios, and in places your development team are familiar with. Later on in the series it would be best to bring all of the favorite locales and new locations to multiple discs, or one disc if possible, for players to play from the disc or hard drive.