Maisie is Playing a New Character

According to Steven Moffat at the Television Critics' Association press tour, Maisie Williams is not playing a returning character. He said,

Once you see what she’s up to you’ll appreciate what a clever idea [the role] was. It’s a significant role, we’re not just throwing her away. It’s a great part and she’s terrific in it. One thing I think is worth saying, because it keeps coming up as an issue, is that she is not playing a returning character. She’s a new character, she’s not a character from the Doctor’s past.

Source: Doctor Who TV