So what will be the answer? What will we learn about the Doctor?
The inherent problem here is that the answer can be anything, but not everything. The answer will provide some insight into the past 50 years, yet also impel the series on for ? more years.
So what's that answer? I really don't know. The answer is not knowable because we don't know where they want to point the series going into the future. That answer is no more knowable than predicting the weeping angels before "Blink" or River Song before "Silence of the Library." The answer will be a new thing.
We know that the answer won't be "Rassilon." Not only have we met Rassilon, which means that he isn't the Doctor, but being a SOMEBODY doesn't actually answer the question of Doctor Who. If the doctor is to be somebody, he must be somebody with a meaning, and it is that meaning which is important, not some spiffy title.
He could be the son of Rassilon, but what does that really mean? That's a title without meaning. Learning that conveys nothing.
"The First TImelord" is not a possibility. We know that the Doctor grew up in the Timelord traditions.
Which leads me back to "something new". I'm sure that there is some fine mind out there who might guess, but that mind will not be mine.
"Run, you clever boy. And remember." So, wacky theory. Clara was created by the Doctor to cause him to remember, then hidden away. He caused himself to forget something about himself. Now, it's time for him to remember. So even the Doctor does not know the answer to Doctor Who. Solving the mystery of Clara becomes solving the mystery of himself.
I can't say that I buy that, but it's a good premise for a series of TV episodes, which makes it more viable than it would otherwise seem.