I have one idea I want to share - to make a mutants in game. Yes, it is snatched from XCom3, and you all know it - victims of alien experiments. Their number will be limited and they show themselves in very special conditions - like if you not going to the mission for too long aliens not just kill all civilians, but make a few of these guys. They can just walk around the battlefield like civilians or even fight for aliens, but once captured alive they can be healed and fight on your side. Since there is no psychic powers in the game (and no need it IMHO) main difference will be penalty for using human technologys and bonus to alien technologys of any kind. And maybe they are physicaly weaker, but naturally better shooters. In game they'll have a black eyes (make such skins is not easy but very easy), and special camo I guess, so you won't mistake them with normal soldiers.