Ros doesn't exist for any storyline reason. She exists for one and only purpose: to strip. When/if she grows tired of stripping or becomes too old to make virgin teens horny she'll die (no doubt naked) and be replaced with another stripper. I don't understand why so many people keep discussing the storyline value/deeper meaning of these T&A scenes. Love them of hate them at least be honest with yourself about what they're there for.
Silly and fairly sexist thing to say, ofc she is there for story line reasons; just because you don't like the reasons or the storylines doesn't mean they magically aren't there.
I hate to say it, but I think they will use Ros for something like this. GRRM has stated that he has told D + D his general endgame ideas, this could of included some things from book 5 before it was published so they could of been planning using Ros as a Jeyne type thing from the start. I don't like the idea of it, but it would make the character make way more sense overall.
I don't think her being Arya would work, but if they did away with the Tyrion / Sansa marriage her being used to be Sansa would be semi reasonable. I do think the imposter Stark daughter story is a bigger thing in the books then the Tyrion and Sansa get married but they don't consumate it and it really doesn't do anything but further get Joffrey to antagonize both of them.
I don't want this to be what they do, but I could see it happening and it would explain some of the so far confusing choices made on the series.