I get what you guys are saying and normally I'd agree, but the reason why I don't think it will be Arya is that the show spent a ton of time setting her up. She's a faceless man, tons of opportunities there....Her connection with Jacquen Hagar and the faceless men, they're up to more stuff in the book and I imagine they show up again....Her Dire wolf has set up a giant wolf pack, I can't imagine that doesn't get used at some point in the story.....
And Sansa....well she's got nothing to add to this story.....they could no longer refer to that character, like at all, and the story would be fine....if anything, a dramatic death would be her most memorable contribution aside from the rape...
I'm not saying Arya lives to see the end of the series, just saying she's got plenty to contribute before it's all said and done....