Balance of Power is the only expansion pack for XvT, released shortly after it, in November 1997. It features 15 excellent and challenging story-driven missions for each faction. They can be played in cooperative multiplayer with up to 8 players or solo. The missions are crafted very well gameplay wise, feature excellent speech actors, who might be familiar from the movies and even introduced a craft not available in the original game, the B-Wing.
If you seek other players who like to play one of those missions, check BSC for the next Campaign Ops Mission War, where cooperative or versus missions are carried out. Usually this event takes every month and lasts for one week. The participating squadrons usually are TFA, TRA, NIR, RS.