The simple answer would be the rest of the DLC.
The expanded answer would depend on what you like. Each DLC adds different things to the game.
Crusade expansion adds the Citizen system, espionage, changes how invasions work, and the Terran Resistance, Onyx Hive and Slyne as playable civilizations.
Intrigue expansion adds governments, crisis events, the galactic market where you can sell and buy resources.
Mercenaries expansion adds the Galactic Bazaar, where mercenary ships can be recruited, and Torians and Arceans as playable civilizations.
From the smaller DLC there are the two free ones, the Map Pack which adds a map editor and several premade maps, and the Builders Kit which adds more cosmetic parts for the ship designer.
There are DLC which add more civilizations to play with or against, Revenge of the Snathi, Rise of the Terrans, Heroes of Star Control: Origins, Villains of Star Control: Origins.
The rest add extra stuff that can impact a game:
Mega Events - several mega events, like Peacekeepers, which sees a bunch of ships appearing and attacking any civilization that is at war
Precursor Worlds - adds special planets with bonuses and new colonization events
Lost Treasures - new colonization events that can reward planetary improvements or ship components
Altarian Prophecy - adds a new campaign
Mech Parts Kit - add new cosmetic ship parts
Worlds in Crisis - more crisis events, planet types