Tell me you know nothing about asoiaf without saying it. There are very good reasons why the fan theory exists. First, the current 3-eyed Raven in GOT is BloodRaven...who is Daemon's great-great-grandson. Like Daemon, BloodRaven was a very "colorful" character, loved by some and hated by others. And, like Daemon, BloodRaven wielded Dark Sister. And there are other parallels.