Honestly im against intelligent or sentient alien lifeforms, an wolf like intelligent hunting pack that actually coordinates with each other and reacts would be nice, but no Empire. IF there are alien sentient lifeforms i hope they get restricted to stone age or even earlier and not real civilizations. That said and Alien AI (not lifeform) that inhabits a planet/moon/asteroid wathever, wether it expands or not could be fun and engaging. Take for example the mining drones from stellaris, they sit in their system and (presumably) mine asteroids all day and shoot you if you get to close. If we can find new tech through such encounters i think it would benefit exploration gameplay and surveying and add a lot of depth to the game.