I'm just curious the 12th (13th) Doctor regeneration was explained. Does anyone have their own theory on how the পরবর্তি regeneration will be explained?

The explanation was প্রদত্ত in the most সাম্প্রতিক special episode, called The দিন Of The Doctor (I think. All of these "...of the Doctor" titles can get me muddled). He had run out of regenerations and was dying. Clara spoke through a slit in time and মহাকাশ to Gallifrey asking the other timelords to help him. They sent him spare regenerations. Most অনুরাগী are assuming he's got another full set now.