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I know the original games now even out yet. But it be silly to think the expansion is not gonna be Korea. It's really the only place the Japanese ever invaded in big numbers apart from Okinawa during samurai times.
You then have the Korean Turtle boats under Admiral Yi which should make for interesting naval battles. Also the chinese can come to Koreas assitance like they did then.
So to me logically it's the only way they can expand the game beyond the boundaries of Japan,