As the final battle cutscene came to a close, the Commander is pulled from his Avatar by those aboard the Avenger, and it's quite possible that the remaining ethereals utilized their psionic power on the Avatar's body to not only cling to life, but find a way to rebuild and bring the fight to XCOM in the coming years. Truthfully, in the closing seconds of the game XCOM gave the ethereals exactly what they were seeking: a powerful avatar to host their bodies. It's entirely possible this gaff by B...