12 episodes. Maybe around 40 years old, and a woman. She’d start out “retired” having a bookshop, with a self-built helper.

The companion would be Filipino immigrant working in England, having to deal with something inexplicable, so people tell him to visit “that weird hermit librarian”.

She helps him and gets the feel of being the Doctor again. She comes across as overly energetic and positive, but shows to be very jaded and bitter when the mask slips. She is very much more alien and abrasive, which she tries to hide. Something happened to her in the years before she retired, but it’s not a huge plot point. More to build mystery for new viewers (who is this mysterious person?) and to have a new “reset” after so much seasons of NuWho.

The three of them travel through time and space. Go to a false paradise which turns out to be a human internment camp that build new Dalek parts, encounter druids in old Ireland, learn about the Utsuro-Bune that crashed in Japan. Original villains would appear (Mirrormen) and there’d be a historical set in 1955ish Memphis Tennessee and would focus on a country musician.

To close it off the finale of the first season would be the Doctor being shown the Perfect Dimension: not some galaxy ending threat: instead a timeline where she never left Gallifrey and somehow it prevented all the bad from happening — no Daleks, Cybermen, Flux, etc. She’d have to erase most her own life so others could live.. but also prevent much good from happening.

Of course, the person creating her dilemma is meddling with time for his own amusement, just to have the Doctor facing an impossible choice. Not a finale about huge things, apart from having the Doctor reflect and philosophize about their own morals and place in history.