Although not, sadly as a companion. Yet. The BBC reported this morning that Maisie Williams, star of Game of Thrones, will step into the TARDIS as a guest star for the currently-filming Series 9. There's no details yet on what role she'll play, but Stephen Moffet did have this to say about it:
We’re thrilled to have Maisie Williams joining us on Doctor Who. It’s not possible to say too much about who or what she’s playing, but she is going to challenge the Doctor in very unexpected ways. This time he might just be out of his depth, and we know Maisie is going to give him exactly the right sort of hell.
When asked for her own thoughts, Maisie said:
I'm so excited to be working on Doctor Who as it’s such a big and important part of British Culture. I can’t wait to meet the cast and crew and start filming, especially as we’ll be shooting not too far from my home town.
It's an exciting development for sure - Maisie has been fantastic on Game of Thrones, and it'll be interesting to see what she does with the more lighthearted fare of Doctor Who.