He's a future Doctor. I don't understand the questioning. The only problem is we'll never see it, because it's probably quite out there and Baker probably won't still be able to play the part. Or- after Capaldi, he might want/need to relax a little and take on this old favorite face for a few years.
And by the way, The Doctor was appointed Curator of the Undergallery (or whatever it's called). Did this take place down there? If it did, it could only be a Doctor who was curator.