I played Sins of a Solar Empire I for years and loved the best complete mod for Soase I-rebellion in my opinion, the Maelstrom mod:
in addition to the base game + 3 new races(Norlamins, Replicators, Trade Alliance) x2 = 6 new exciting factions + new playable pirates faction + some interesting changes. This mod got me so excited that I had the whole StringText translated into German.
It would be great if there was a mod like this for Sins 2, because it would allow a whole new gaming experience with 13 factions.