“The Doctor will meet a new friend in the most unexpected place.”
“Who she’s playing, how the Doctor meets her, and even where he finds her, are all part of one of the biggest mysteries the Time Lord ever encounters. Even by the Doctor’s standards, this isn’t your usual boy meets girl.”
Which is what we know. So, naturally, I’m going to take that as a challenge. This will, coincidentally, lead to some companion character ideas. Some more viable than others.
1: The Doctor’s opposite sex clone. Silly idea already suggested. Gets complicated when one regenerates - when the other one regenerates, do they cast someone who looks like the new version’s opposite sex clone? Or handwave “I never know what I’ll look like...”? Or for added amusement, the Master’s opposite sex clone.
2: Romana. Duh. Or equivalent Time Lady, reality has been slightly cracked so there’s space for one to get out here and there, surely.
3: Someone from 1963. Because she’s the 50th Anniversary Year Companion, this makes a fair bit of sense.
(Being from Coal Hill might be a bit much. Being Susan definitely would.)
4: Someone from an earlier time period. Easy enough to be a POV character if their mindset isn’t too foreign - the 60s would work for that, the high Middle Ages less so. For added complications, she could be a Celebrity Historical - Mary Shelley’s doing it in the Big Finish series, after all.
5: Someone from the future. Similarly, keep up the “it’s like now but with spaceships” portrayal of the future and it wouldn’t be that big a deal, just the odd joke about improbable-sounding events from the near future like Lord-President Cowell. Might have a blaster.
5 a: A huge continuity snarlup. River and the Doctor’s impossible alternate future daughter. Something like that. A walking paradox - this becomes part of the arc of the season, time goes wrong all around her.
6: An alien in human form (other than a Gallifreyan). There were four of them in the TARDIS in the early 80s. Optional shapeshifting or other superpowers. Maybe not a Carrionite.
7: A less disco Movellan.
8: A non-humanoid creature. I hae my doots here, as pre-reveal publicity mentioned she was pretty specifically. A near-human Green-Skinned Space Babe like Majenta Pryce could work, though. Might be a way to go if there’s a second companion, though, like an unflappable Ood butler or something.
9: An escapee from the Land of Fiction. Might be a bit too meta for an ongoing thing, but having Elizabeth Bennet in the TARDIS would be rather amusing.
10: An intangible transphasic being from a now-paradoxical universe. Or “a ghost” for short.
11: Grace O’Malley, Ireland’s Pirate Queen.
12: Liz Shaw.
13: Some ordinary person from Britain in 2012. ... Naaah.