First off, they complain that he's too old. Those babies should just be dismissed with a wave of the hand. Preferably with a painful smack against their cheek.
But they also complain that Peter Capaldi has already been on the show, and that he's also been in 'Torchwood'.
O'Bviously these are "fans" who never watched the show prior to 2005. And even then, most of them don't even bother with Christopher Eccleston's incarnation and focus only on David Tennant's and Matt Smith's tenures as the Doctor.
But if they knew their Classic Who history, they'd know that at least one Doctor met the template for his future incarnation - the Fifth Incarnation of the Doctor was shot by Commander Maxil of the Gallifreyan Guard. And it's not just confined to the Doctor. His Companion, Romanadvoratrelundar, better known as simply Romana, was a Time Lady of Gallifrey who met Princess Astra of Atrios during one of her adventures with the Doctor. And while later selecting the imprint for her new regeneration, she finally settled on the form of Astra.
Every time a Time Lord regenerates, he (or she) is "reborn" with a whole new physical appearance. The simple splainin could be that their body's DNA just re-arranges itself to become something totally new, but the basics are always still there.
But because of these previous encounters with characters who would one day look exactly like them, I think there is something else involved in the procedure.
Without them even realizing it when it happens, I think a Time Lord's body absorbs DNA from others and keeps it in storage until such time as when a "reboot" is needed. On some basic genetic level, I think a Time Lord's body recognizes when it comes across the template for a possible candidate and only then takes in the necessary DNA for the next incarnation.
So over the course of the day, I'll be suggesting how this process came into play with each successive Doctor. Along with a couple of side trips for Romana and the Master......
And as you might expect from me, it's all an excuse to make theoretical connections to other TV shows!
I've written about this topic before, but never got around to finalizing all of the candidates for the DNA donations. So I'll be doing that throughout the rest of the day.
The TARDIS Data Core Wiki has an article on all the intricacies of regeneration. And I don't see anything that could really derail this theory.....