The Maelstrom Mod introduces four new races to the Sins of a Solar Empire Rebellion Universe. They are independent of the original races and have unique strengths and weaknesses. Maelstrom is available for Rebellion, Diplomacy, Entrenchment and Original Sins.
ship designgs are closest to original sins, just bigger and more detailed. Also the models are more logically built compared to sci fi space ships than most mods. My new favorite mod