Enclave are dead as a Faction.
I don't know. Perhaps not as dead as you think. FO3 showed us that they had regrouped to a certain extent after F2, and why wouldn't they? Ideologies tend to not die, so much as fade, and ebb, no matter how much of a defeat they've had. Look at the Nazis. Convincingly thrashed at the end of WW2, yet there are still people out there who believe in those ideals. Get enough of them together, and all of a sudden they start to become a power....
Finding "pureblood" humans might be a challenge, but as with all ideologies, it would morph and evolve as time goes by. Genetic screening results within certain parameters would be enough, after a while.
If it's written well, I would not be averse to seeing them as a faction within future FO's, though my preference would be on new protagonists.