One of modern Doctor Who's biggest loose threads, the fate of Jenny (Georgia Moffett) - the Doctor's Daughter has never been resolved. The last time fans saw Jenny in "The Doctor's Daughter," she was piloting a ship away from Messaline - to forge her own adventure in space. Jenny may not have been conceived the old fashioned way, created near-instantaneously by a progenation machine but she is the Doctor's biological daughter - complete with two hearts and Time Lord DNA (meaning that, technically, she should be capable of regeneration).
Jenny might seem like an obvious choice but she's not a sure-thing - since she was stranded in the far future, separated by thousands of years from the past Williams' character appears to reside in. There's no guarantee that Williams' interaction with Capaldi, teased in the trailer, actually occurs in the past but, nevertheless, a recent past setting seems likely. Jenny might be a Time Lord but she doesn't have the TARDIS.
If Williams is playing a regenerated version of The Doctor's daughter, she'll have needed to procure a time vortex manipulator or other time-travel tech in order to resurface in the past. Given that Jenny was last seen in the future, where humans have access to advanced tech, it wouldn't be too much of a stretch to explain.
The Doctor's daughter is, without question, a storyline that many fans would like to see re-explored - especially considering the Doctor has now outlived his actual children by centuries. For that reason, it would be interesting to see the Time Lord finally reunited with his own flesh and blood - the GENetic anomalY he believed was dead? After all, since it was Steven Moffat who suggested that Jenny should survive events of "The Doctor's Daughter" back in 2008, it's reasonable to think the showrunner might still have ideas for the character.
One of Doctor Who's original cast members (played by Carole Ann Ford) Susan Foreman is the Doctor's granddaughter. Audiences were first introduced to the time-traveling Doctor, when Susan's school teachers follow her home after class - and discover the TARDIS (a name Susan bestowed upon the time machine). Susan traveled with the Doctor for 51 episodes, 10 stories total, before she fell in love with a human soldier, David, and settled down with him in 2164. The two marry and, aside from a few one-off cameos in anniversary specials, the Doctor never sees Susan again; though, it is implied that Susan lived a happy life with David, using age make-up to hide that she didn't age as fast.
Episodes in the modern series assert the Doctor's family members are all missing or presumed dead (further complicated by events of "The Day of the Doctor" which indicate Gallifrey wasn't destroyed) but Susan's fate was never confirmed on screen. Susan is a full Gallifreyan - meaning that she could easily have regenerated (into Williams) after David passed away.
Considering how much the Doctor broods about being the last of his kind, it's hard to imagine the Time Lord wouldn't have thought to track Susan down. Maybe he never searched Susan out for fear that he'd put her in danger but, whether or not he believes she is dead, it'd still be easy to bring the character back and explain where she's been in the interim. Like Jenny, Susan is an unresolved thread - one that Moffat might enjoy revisiting.
- The TARDIS - In "The Doctor's Wife," the TARDIS took on human form when it's consciousness (aka matrix) was absorbed by a human host, a woman named Idris. Depending on the circumstances, it's possible that Williams could be playing a similar manifestation of the TARDIS matrix.
- Clara - While versions of The Impossible Girl were spread across space and time, in order to save the Doctor, there are also plenty of reasons why the Doctor might visit his companion at a younger age. Yet, since much of season 7 and 8 was about Clara (the Impossible Girl mystery and Danny Pink romance), viewers might be ready for the focus to shift elsewhere for bit. Not to mention, Williams and Capaldi's exchange implies a history with the Doctor (and larger implications) that a younger Clara simply would not possess.
- The Rani - A malevolent lady Time Lord (Kate O'Mara) that values scientific research and the pursuit of knowledge above all life. Last seen in 1993 ("Dimensions in Time"), The Rani's fate remains unknown but, given that The Master and The Doctor have both managed to cheat death, there's no reason to think The Rani couldn't have also survived - eventually regenerating into Williams' character.
- Romanadvoratrelundar (aka Romana) - A fellow Time Lord, Romana falls into the same category as The Rani and Susan. If the Doctor and the Master truly are the last of the Time Lords in this universe, then the showrunners would need a clever explanation for how Romana survived (and hid) - not to mention why she's resurfacing (as Williams) now.
- The Doctor - The ultimate twist. What if Steven Moffat cast the next Doctor years before she will take the starring spotlight - and introduces the Williams Doctor in her past (the Capaldi Doctor's present)? Easily the most far-fetched honorable mention on the list but, given Moffat's penchant for mind-bending time-travel stories, debuting a Williams Doctor long before (and possibly more than one regeneration before) she takes on the titular role, would be an incredibly ambitious idea.
Those are just a few of our explanations for who Maisie Williams could portray in Doctor Who season 9. Sound off in the comments section below with your own theories!
Doctor Who returns on BBC and BBC America September 19th 2015.