You know reading that gave me a kinda wacky idea. Since an Imjin expansion would take place after the main game's time period, I wonder if it would be possible for CA to include some kinda save game reader. One that checked which clan you won most with and have that clan be the unifying force for the Japanese Invaders and their family members as heroes and generals. So if you spent the majority of your time playing Hojo, the Hojo would be leading the efforts.
I love this idea! :D Talk about truly molding the fate of Japan (if you opt to play as them). Even allowing your ending infastructure and current armies that were developed during the SJ campaign to continue to be used during the expansion campaign. Though now that I think about it, if you don't start from scratch, the expansion campaign might be a little too easy. Keep your heros anyway, if you like that sort of thing.