Well it's one thing to make an expansion, it's another to make a new game with all new storylines. And that's what an expansion like that would become: and all new game.
See the thing is you have to keep most of the original units (if not all), which means it's still rebels vs empire at heart. You have to remember that strange things will happen, like obi wan still running around, yoda still kicking butt, death stars running amok blowing up planets etc.
I don't think the next expansion has to be complicated enough to contain a totally new storyline. I think it needs new units, like Lando Calrissian and Princess Leia, of course new space units, and a campaign that won't contradict any previous storylines.
I'm wondering, after finishing the FOC campaign, if we will be getting a new faction, or if we will just be getting more units for the existing factions? You know the storyline will have something to do with Silri finding those frozen warriors. But will they be part of the empire? Or a new faction? Personally I don't want to see 4 factions since the newest one is guaranteed to be so overpowered and so out of place within Star Wars (like I think the ZC units are) that I won't want to play it.