I mean in the books it was clear it doesn't matter what fake Arya looks like, it just needed to be a northerner that knows about Winterfell since everyone is dead. They can use it as a way of just showing that Bolton and Ramsay know she's fake while sorta making tortured Theon more important again too. (heck they even had that scene with Ros breaking down in front of Littlefinger... like she would do again with Theon) There is also history between Ros and Theon from back in season 1 - I think it would make her wedding to Ramsay that more intriguing and show that Theon is so scared of Ramsay that he's willing to not really help Ros out (immediately). In the Tyrion chapter I reread it was said that Littlefinger found Arya (and gave to Tywin... though Tywin knows she's a fake) and at the moment in the show Ros is the only one who makes sense there, but I suppose it could be someone new... It just seems highly likely to me that they just might implement that role on her, but I guess we shall see in time. I'm willing to put a golden dragon on it I'm right though. :D
