How Arya plays into all of this really is a good question. I don't think she kills Cercei, though. It seems much more fitting for Jaime to kill her, and as I mentioned it basically wraps up his character arc nicely.
Was there the valonquar prophecy on the show, though? I know they had the gold shrouds part of her prophecy, but I was thinking they left off the valonquar portion of it.
I guess the show could go in a different direction. I think Jaime is a lock to kill Cercei in the books, but it is worth noting that the show hasn't really portrayed their growing rift at all. In the books, that's a huge part of Jaime's character arc. On the show, it's nonexistent.