Darth Lucas said:

I have a theory. Everybody is talking about the hooded figure speculating he may be an ancient warrior of some kind. What if this character turns out to be Darth Plagueis? He actually DID manage to save himself from death and has been in some form of hibernation for years and years. The shot from the trailer is him waking up and igniting his lightsaber ready to kick some ass. The force awakens. As in the ultimate user of the force awakens. Huh? Huh???

I know it's dumb but I love speculation like this ^-^

I'd prefer someone more ancient than Plagueis. How about Darth Bane, the founder of the Sith, or a being from before the founding of the Sith?