So, I have a bunch of these (I just kind of idly come up with them). I pitched one in another thread here:
I have another idea where the Doctor ends up stuck in the Next Universe (after the end of this one), with all new rules and monsters and (almost) no old stuff. The Doctor’s backstory is now simplified to “an eldrich being from before the universe”.
The first season sees them exploring and revelling in the newness and alienness of the new universe. The writers will be explicitly told to make it weird. We’re making up the rules here, we want to do things the (already broad) show usually can’t. Maybe there’s a religion with a presence on every civilized world, even those who haven’t made first contact yet. Maybe on January 1st, 5137, a flux-style event called The Inversion took place, and all of space was turned to water and every planet was turned inside out, becoming a massive atmosphere bubble, civilizations clinging to the surface as they drift through the deep blue sky, following their old gravitational paths around their impossibly burning suns.
I think the first season finale sees the doctor get lonely/sentimental and basically build a replacement Earth. They save a dying species that looks close enough to human, and drop them off on an empty planet that looks close enough to earth, then guide their development until we get a planet that’s close enough to modern earth that we can set stories on it and have them be relatable to a modern audience. This also means we can threaten to destroy the earth and mean it. Of course, it also makes the Doctor basically the creator of this earth, which is something I think we can do cool character stuff with (They don’t want to be a god, they just keep accidentally fitting the definition).
With season 2, the Doctor has now made a name for themself in this new universe. They take on a companion. They’ve started to make enemies. I think we start to tease the presence of another powerful being from before the universe. A few episodes in, we reveal it to be Clara (ideally, actor depending, definitely someone we know). The Doctor is surprised at the friendly face, but welcomes it (side note, the Doctor introduces her to their companion, so new audiences don’t need to recognize her). The 3 of them travel together for a bit, until the mid-season finale, where Clara betrays everyone and kills the Doctor, revealing herself to be The Master, as played by Jenna Coleman.
The next episode picks up with the companion on their own vs the Master. The Doctor doesn’t appear. The episode after that picks up with the Doctor in the afterlife, because apparently this universe has one (tying in with the mysterious universal religion from season 1). Once again in a new environment, the Doctor has to quickly learn the rules of this new spirit realm in order to survive (well, you get what I mean). I think they don’t get out right away. I think the narritive bounces between the Doctor in the afterlife and the companion in the real world, until the finale where the Doctor comes back from the dead to save the earth, saying out loud the words “Deus ex machina”.
I don’t really have any plans to take the doctor back to the old universe. If another showrunner wants to, that’s on them. I haven’t destroyed the old toybox, just opened a new one.
As for spin-offs, I propose a variant on something I’ve already pitched elsewhere, but I think it works better here: An anthology of mini-series, with each season covering a different historical event in the new universe, maybe something detailing the lead up to/aftermath of The Inversion, for example. Sometimes you can have the Doctor or the Master show up for an episode, sometimes a character established here will show up in the main show.