Star Wars: Imperial Assault – Heart of the Empire
Buy this firstA Reddit owner recommends Heart of the Empire as a self-contained, tougher campaign for the Rebels, and BGG users rate it 8.51 across 766 ratings, the highest of the big boxes after Jabba's Realm.
Needs: Just the base game
Adds: A 16-mission Coruscant campaign with three heroes, the ally Ahsoka Tano, 13 other figures including an AT-DP, over 130 cards, and energy shield, rubble and power tokens.