To me, the most likely place for a Suikoden VI setting would be Harmonia. They have already given us a history of the country, not to mention several characters throughout the game with direct connections to the country and government itself. On top of that, it is, from what I remember, the oldest nation according to the current timeline and has been mentioned to some degree in every Suikoden game to date (conjecture, but I recall a small mention in each game at least). It would bring an immense amount of information to the forefront regarding the entire Suikoden universe regardless of whether or not it was a prequel to IV or a sequel to III.

I would play any Suikoden game released regardless of setting, but a Harmonia-fueled game would have me incredibly excited.