I think the best idea for an expansion would be to keep the story elements that have so far been developed, but make the campaign much more interactive, engaging, and involve the player in much more character development, while allowing us to continue the Star Wars story approximately as we know it.
For example, I think it would be awesome to see an expansion that starts the player as an Imperial officer in the post-Hoth time period. After some standard anti-Rebel and anti-corrupt missions, the player could become involved in the Zann Consortium (either as a victim--for instance, through a bribe to the player or as a result of some extortion--or willingly). There, the player works to extend Zann's influence during the time that Tyber Zann himself is off chasing the Eclipse. When Silri gets ahold of her whatever-they-ares in the Emperor's vault after the Battle of Endor, the Consortium could dissolve into infighting, splitting into at least two factions (Silri vs Zann), or possibly three (Silri vs Zann vs player). In the process of defeating your rivals within the criminal underworld, the player would then reach some sort of decision (or, possibly, be forced) to join forces with the Rebellion and the final stages of the campaign would be a very open-ended exercise in sweeping away the last remnants of both Zann's Consortium and Imperial forces with Rebel troops. Throughout the whole thing, I'd love to see how players approach campaign objectives--which planets they take in which order, for example--affect what happens to them next.
That would be my idea of a good time, anyways. If I can tear myself away from Supreme Commander and Spore.