"What Took You So Long, Old Man?" -Who Is Maisie Williams Playing in Series 9 of Doctor Who?

Since the landing of the Series 9 trailer for Doctor Who, the one question that has been flying around Whovian's heads is who exactly is Maisie Williams playing in the series?

There can be quite a few possibilities here, a younger version of someone, an old companion or someone completely new! Let's narrow the choices down a bit more. Some being obvious choices but could still be true. We never know. Especially for Moffat as we never know what he has up his sleeve.

A Younger Version of a Character?

Could Maisie be playing a younger version of a character? There's a few chances here. One of them being River Song. There's that obvious chance that Maisie could be a younger River Song. If you go back to the Episode "Day of the Moon," at the very end we see the little girl regenerate but we don't see who she changes into. Hmmm. Is this where she regenerates into Maisie Williams' character? We know that a younger version of River Song is Melody Pond. In case you don't remember, Melody Pond is Amy and Rory's daughter. Yes River Song is Amy and Rory's daughter. Nothing stops Maisie from being a younger River, though.

An Old Companion?

Could Maisie be an Old Returning Companion? One being Jenny (the Doctors Daughter). We see Jenny killed in the Episode "The Doctors Daughter". Obviously the Doctor is upset that she was shot and killed. But what he doesn't know is that Jenny came back to life because of Regeneration energy. So was that version of Jenny killed and she regenerated into Maisie? There is that possibility.

Amine Chioukh said this:

"Maybe she's the Doctor's Daughter. Yes, maybe she regenerated herself into Maisie Williams. It would be so cool." - Amine Chiukh

Matthew Lee says:

I want her to be Susan (regenerated into a younger age) or maybe Jenny (from Doctor's Daughter) sort of father and daughter attitude towards the Doctor. I do not want her to be a younger clara or missy. Because I find the story of multiple claras a bit annoying and a younger missy (now a female master) is just too much. - Matthew Lee

Okay, what about Susan? I've seen many people out there saying that Maisie is playing Susan. Craig Arnott saying this:

"Old man" meaning "dad". - Craig Arnott

We know that Susan is the Doctor's grandaughter. But that doesn't mean she can't call him "dad." The First Doctor left Susan on Earth, "One day I shall come back. Yes, one day." So, is this Susan and the Doctor reuniting?

Clive Barlow says this:

It may be Susan, as at Comic Con Peter Capaldi said he wanted to see his granddaughter again. - Clive Barlow

James Swadling has an interesting theory:

"Could be The Rani, Romana, The Doctors Grand daughter Susan even though did not finish the academy so technically not timelord but this is Moff and plot holes, could even be and i hate to say it, but what if its The Doctor, and he has met himself in the future and has gender changed, and i hope to god that they didnt do that, but THIS IS Moff. or could be another regen of Missy (the Master). or Cassandra found a way lol, or even just a Zygon" - James Swadling

How About a Brand New Character?

Louise Rogers thinks so:

"I reckon she's returned as someone we've not seen before and she's a new character but a flash from the Doctor's past". - Louise Rogers

Is there a future companion on our hands here? Who is to say that Maisie Williams is a new character and will jump on board the TARDIS with the Doctor and Clara? Or will Maisie be replacing Clara? Is Clara actually leaving?

What If Maisie is an entirely new character that we've never seen onscreen before - perhaps the Doctor has met her before, though. The Doctor seemed to have recognized Maisie's character, which could be significant. It may relate to Louise's comment. Maybe it is a "flash from the past." Could she be a fellow Timelord (well, Timelady) that we have yet to meet before this point? Remember that the Doctor said this in the trailer to Maisie: "You." He certainly said that in a shocked voice. That sounds like it could definitely lead to something.

Okay, say she is a Timelord - you should know what is coming... if she's a Timelord then maybe she knows where Gallifrey is and knows the coordinates of his location. Is Gallifrey finally returning?! I hope so!