I wonder if Sansa might be sent alone as Alayne Stone to Ramsay's wedding ostensibly so that the Arryns can build an alliance with the new Warden of the North. When she gets there, she takes up Manderly's role and starts killing Freys. (Though it'll be interesting to see if there's any confirmation of the Theon Durden theory...) When she's discovered, Theon helps her escape.
Miranda plays the role of Arya Stark. Does she look like her or is she the right age? No, of course not. No one really ever believed that Jeyne Poole was Arya Stark but they put up with the farce because he's got the backing of King's Landing, an actual army and no genuine alternatives are present.