The expansion to Star Wars: Empire at War features an entirely new faction called the Zann Consortium along with a new story campaign for it. The Rebel Alliance and Galactic Empire also get new units and heroes to use against each other, and the new underworld faction. All three factions are able to battle across the galaxy on or in space around the original Empire at War planets and thirteen new worlds.
The gameplay has also been improved and features larger maps, more depth to land battles by having new unit specific terrain and strategic locations along with the ability to customize your ground bases in Galactic Conquest.
Don't have the expansion, but couple years ago got Best of Star Wars collection that has Empire at War itself.
Haven't played much, I haven't really gotten into RTS since I 20 years ago played Dune 2 enough to be done with the genre...