Rixao Zendial
*THIS POST CONTAINS SPOILERS FOR SEASON 6*
I've always wondered if they might use River's 'transfer' of her regenerations to The Doctor as a way to explain it. Would it mean he has a whole new cycle? Have the number of regenerations that River had left? It's certainly been something a lot of people have pointed fingers at as a reason to continue his regenerations on.
I'm wondering more about The Valeyard. He showed up during the Sixth Doctor's tenure, and even though they had a good plot hook for him, he never did show up again. I've always hoped that they might bring him back in an effort to explain The Doctor's growing 'inner darkness'. Perhaps The Dream Lord was a prototype Valeyard?
The whole River regeneration transfer thing is certainly on the list of things that could be used explain additional regenerations. But i wasn't sure how many times she could regenerate as she is a human with time lord DNA, I'm not sure how much of her is human and how much is not. But it would be a easy way to get 9 or 10 more regenerations
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(Shamefully) i completely missed the parallels between The Valeyard and The Dream Lord. But your right The Valeyard was a really interesting character which, as i remember, was very quickly abandoned. It would be great to see him making a return in some form. The BBC do like to bring old characters back.