Maisie Williams of Game of Thrones fame is on the set of Doctor Who. Maisie plays Arya Stark, the most popular star of GoT by Twitter standards.
It isn't known what role she'll play or on what episode, only that she is a guest star and that Steven Moffat says, "We're thrilled to have Maisie Williams joining us on Doctor Who."
A listing on IMDB shows her as being in episodes 5 and 6 which are said to be period episodes called, "The Girl Who Died" and "The Woman Who Lived." Hopefully it will be period with Strax, Lady Vastra and Jenny.
Hopefully we'll see more details before August, when the show will most likely be back on air, though it hasn't been announced.