This could be a reason for the Billie Piper doctor situation (no 1) And this is a storyline idea for the 2026 Christmas Special (no 2)

1. My theory is that reason that the 14th regenerated into the same face as the 10th doctor was created by the metacrisis doctor in the alternate universe with Rose to try and get back to the actual doctor's reality again, and the 14th doctor that you see is actually the metacrisis doctor who used the concept of regeneration to help get back to the actual doctor's reality which would explain why the 13th doctor's clothes changed when she was regenerating. This would also explain the concept of bigeneration, after the metacrisis doctor fused with the regenerating 13th doctor, it meant that there were two beings inside of one, which would cause the next regeneration cycle to actually be the separation of the two beings (the metacrisis doctor and Ncuti's 15th doctor) this theory would mean that if not interrupted, the 13th doctor would have actually have regenerated into Ncuti's doctor. But getting back on topic, now that the metacrisis doctor is back, it was Rose's turn to use the same method which is why we see her as the new doctor, its actually Rose using the process of regeneration to get back to the actual doctor's reality. This could suggest that the 16th doctor actually is Billie Piper and the next regeneration cycle will be a bigeneration cycle due to their being Rose and the new doctor in the same body.

2. The new christmas special should be where the doctor's biology has been messed up by being around so many of the gods of the pantheon's domain. So the doctor has lost his/hers identity and keeps constantly jumping through his past incarnations and companions: Rose, Bill, Yaz, Martha,12th, 11th, 10th, 9th, 8th, etc. And for each time he jumps between them the current doctor that is on screen gets a while of screen time and gets to be their doctor again. The tardis should land on Christmas day 2026 in a panick and kick the doctor out because it doesn't recognise them as the doctor. So the doctor is on earth and scrambling around trying to find out who they are and if they are even the doctor. (Jumping to former incarnations as the episode progresses) When the doctor jumps to his 1st incarnation (played by Bradley James), he should bump into Susan who is walking around town. She should explain that she sent a psychic message to him because she new that this was going to happen. She and him should walk around the festive town and chat about all the good times that they had, and after that conversation, the Doctor should feel like himself again and that hes gotten his identity back. As he realises this, he jumps back to his 15th incarnation. The episode should then come around to a close with the doctor (having jumped back to the fifteenth doctor again) finding their identity after chatting with Susan. He walks back to the Tardis as Susan says "we will meet again" They then walk back into the TARDIS and happily regenerate into the next doctor. This story would not only explain the Rose situation, but would also give a satisfying story to Susan and fix the lack-of-Susan ending in the reality war. It would also fulfil the ideas said in the leak where it read that they wanted to get as many doctors in this special as possible.

Final note:

So what do you all think, is this a good theory and is the storyline good as well? Let me know :)