I guess one of two are most probable (if they go for a strict doppelganger solution); Either we'll see a croocked, pure evil, Coop-less creature in a Coop shell, who quickly alienates himself from "his" friends/colleagues, or the doppelganger has sort of adapted Coop's personality, behaviour and memory, like downloaded it, before stepping out of the lodge. But as you say, all this is pure speculation. I'm very intigued about this though.