I greatly enjoyed playing through xenonauts, and I think one of the things that set it apart from similiar titles and made it more interesting were the tactics and unique properties each of the enemy types possess. For example, the androids heavy armor requiring penetrating weapons, and the way they would use their innate protection to ignore cover and rush you.
On that note, I really like your idea with the sebillians. Perhaps consider a terror unit that really embodies regeneration fully, requiring massive firepower to really kill it off. It might not be the best idea to make a basic unit like sebillians too hardy, but I like the concept of taking one down from long range and watching it slowly return to fighting status as it picks itself up and it's wounds close, unless you finish it off. Combine that with heavy armor on the officer and above units, and you really feel like your fighting a near hopeless war.
