3. He Is A Mentor/Namesake/Father figure
This theory purports that this "Doctor" is the Doctor in title only. He was a mentor or someone that the title "Doctor" was handed down from to our "Doctor". He did something that caused him to give up his title to our hero. He was a father figure and someone our Doctor respected, until the event that caused the title to be passed on. Likelihood? Maybe. This would be unexpected and feel a little like cheating. I could see Moffat doing it. Ha! John Hurt played a great mentor in the Hellboy movies, and it would be nice to see that again. On the other hand, what the heck would he be doing in the Doctor's time stream? But Clara isn't the Doctor either and she got in there.