Doctor Who: 10 Storylines That Will Never Be Revisited
7. Tasha Lem's Connection To The Doctor
The fact that Tasha Lem only appeared in a single episode (The Time of the Doctor) means that there wasn't really a lot of time available to fully develop her. That's fine of course, because she adequately served her purpose in the time she did get, but it does mean that there are a lot of unanswered questions surrounding the character.
As was the case with Adam, under normal circumstances, an abandoned side character wouldn't be noteworthy in the slightest. But when we first meet Tasha, it's strongly implied that she and the Doctor have had a close - possibly even romantic - relationship in the past, and what's more, she can even fly the TARDIS.
Because of her flirtatious attitude towards the Doctor and her familiarity with his time machine, a lot of fans think that Tasha is a future regeneration of River Song, which would open up a whole other can of worms when it comes to that character's timeline. Even if she isn't River, it's not like a random woman would just know how to fly the TARDIS, so there's a story here somewhere that we just didn't get to see.
It really does feel like Moffat wanted to explore Tasha a bit more, but never got the chance to do so, for whatever reason. There's even a rumour that The Time Of The Doctor would have been stretched out into a season arc if Matt Smith had stayed for another year, but this has never been confirmed. In all likelihood, Tasha is gone forever.