My story to tie up some loose ends
I am under no illusions that this will realistically happen or that anyone will like it, but this is my idea for what could happen following the end of The Reality War. It ties up a few (but not all) loose ends so sorry for those who wanted it to include >!Ruby, Belinda, Poppy, Rogue, the Pantheon or the Master!<. Yes, it does retcon a few things too but that is part and parcel of the show. With any luck, it's still satisfying.
The Doctor's (Billie Piper) regeneration energy sputters out and stalls. They are struggling to stabilise and flips between faces as reality has been re-written (and quite by chance, within the first 15 hours of regeneration cycle as noted by the 16 hour clock in the TARDIS). They will flip between some old familiar faces over the course of the story.
The TARDIS^(1), recognising something is wrong with the Doctor's regeneration, warns them and states that they have seen someone is playing with this same energy so the Doctor will need to stop that power getting into the wrong hands.
A quick outfit change later^(2), The Doctor goes to the basement archives of the Time Hotel to investigate, and works out they'll need to talk to The Boss to get any answers. They are about to head back to the TARDIS, when they are invited to the Time Hotel's penthouse to meet The Boss.
The Doctor is welcomed by some senior staff who introduce them to The Boss who is revealed to be ...Mrs Flood/the Rani^(3) from the future. Following everything that happened in the Reality War, she's conducting experiments to work out how to re-create Time Lords biology. A lot of time has passed since we last met her.
She is using time sensitive, telepathic and two-hearted species^(4) to conduct her research. In her lab, we can see Ood, Meepkind, Weeping Angels, and many other species in tanks, cells and hospital beds. There is a special section of her lab where she is trying to 'weave together' these various elements.^(5)
The Doctor has another vision of Susan calling out to him, but stronger, more vivid. The Doctor finds a tank with an unconscious Susan in there. Mrs Flood explains that, due to the shifting of reality, she slipped into another timelines and managed to find and kidnap Susan, a Gallifreyan who hasn't regenerated. When she's ready, Mrs Flood will keep triggering the regeneration process until she has collected all the data she needs (i.e. until Susan's dead). She has used her psychic abilities to try and reach out to the Doctor to help her.
Mrs Flood reveals the reason Susan is so special is that she was an oddity among the children of Gallifrey and a lot about her past was unknown. The Doctor had taken her under his wing and kept her away from the Academy and other authorities, but when Rassilon took an interest in Susan, the Doctor (having snuck away from Gallifrey a few times before) decides he should take Susan with him and run away properly (although he removed most traces of Susan from the Matrix on one of his trips back).
Susan comes round, recognises the Doctor and they have a heartfelt reunion. The Doctor, appalled at the experiments, sees who is in the other tanks and also finds an older 14th Doctor.^(6) 14 reveals he was also taken from his future, long after Donna's family have all moved on. The Doctors and Susan try to save the day, sending the lab subjects back to their own time periods. 14 tries to stop Mrs Flood but she is escapes^(7). Meanwhile, Susan is critically injured.
The three of them get back to the TARDIS. The Doctor (now David Bradley) goes to Susan and encourages her to regenerate but she never learnt how. The Doctor tries to coach her but she says she thinks it's too late. They have a final heart-to-heart where the Doctor confesses that all their companions have been trying to fill the gap Susan left^(8) and may never manage it but Susan has been close to their hearts and they've never been lonely. Susan dies and her body turns to stardust.
With 15 hours almost up, the Doctor (finally Ncuti Gatwa again) needs to stabilise their regeneration. They begin to regenerate but the process still isn't working. The 14th Doctor realises it's his turn to play his part. His body (and shadows of other Doctors) merge into the Doctors glow, getting brighter and more powerful each time.
The clock in the TARDIS ticks over to hour 16^(9) and the TARDIS is overwhelmed by the glowing light as the regeneration completes. From outside, we see the light fade from the TARDIS window. A pause. We hear the TARDIS engine fire up, then the lamp on top start glowing, and the TARDIS takes off. Quiet falls again but the adventure continues.
Footnotes and clarifications:
- This could be anyone like the Fugitive Doctor or Oodkind, but it would need to be a cameo that made sense.
- Something a bit more robe-like that can accommodate the different body shapes, heights etc of the various people playing the role
- Yes, I know Mrs Flood has been retconned before into becoming the Rani, so might as well retcon her to be The Boss, a systematic (Rogue mentions paperwork) two-heart curious (as the Meep states) person who is familiar with the Doctor (says "Hi" via Anita)
- She kidnapped a lot of them herself but also used people like Rogue and the Meep as bounty hunters. Other Time Lord abilities are available.
- Might as well tease looms again.
- Yes, Tennant again but looking visibly older, maybe dressing a bit like Wilf. A small role but not much to explain what happens in the gaps.
- Escapes, or is trapped, or appears to die. Doesn't matter, she's gone away.
- A lot of other people have come up with this idea, but I really like it.
- It started at hour 1 so we can end on 16 for the symbolism.