The ending--big SPOILERS!
So, this has probably already been discussed quite a bit. If so, I apologize. But in the end scenes, between Wade and Halliday....Halliday interacts and converses with Wade. So, he can't just be a "recorded hologram", so to speak. A real person has to be behind that avatar....thinking and responding. So....is it Halliday (not dead afterall) in some remote location, living now only via his avatar? Or....could it be a proxy (Morrow?), posing as Halliday via his avatar? I guess my point is...SOMEONE alive and hooked-up to a VR device....is acting as Halliday's avatar. Even Wade alluded to it: "You're not really dead, are you?" And Halliday didn't give him an answer.
Is it supposed to be obvious he's not dead? Or deliberately ambiguous?