The Enclave make great villains, and Sarah Lyons does mention that the Elder, her dad, has them on the lookout for Enclave Reinforcements along the borders. But she was of the opinion, as am I, that they would need a long time to recover from that beating we gave them.
If we face the Enclave it'll likely be whatever was left on the West Coast. That bumper sticker on the eyebot could have just been local - the mom might have sent her kid all the way to D.C. and such.
But I doubt they'll include them in this game, we only just got back from Fallout 3 so a change of pace is in order. Now, the only canon Fallout game without the Enclave was Fallout 1 which had the Master and the Super Mutant threat.
What if, and this is a big if, we face the Master in some sort of reincaration? Some offshoot experiment from the Maripose Base?
Can't wait to find out.