Not the theory I've heard at all.
(SPOILER)It has to be a past Doctor because otherwise how would Matt Smith's Doctor know what he had done?
He's the Doctor between McGann and Eccleston. Only regeneration we did not see. And what awful thing happened between 8 and 9? Time War. NuWho Doctors become 10th, 11th, 12th.
Oh and yeah, sorry, I accidentally called Matt Smith the 12th Doctor earlier in this thread because I found out SPOILERRRRS. I think I played it off well though 