I know the Quiet Isle theory and it gives a nice ending to the Hound, but the tv show needs to streamline and won't have time for that.
I also wonder about the Jeyne Poole as Fake Arya storyline, which is necessary for Mance to sneak into Winterfell and Theon to get his act together and escape.
If they keep Arya travelling with the Hound for a while longer, and have him "die" any way that fits, Arya could take the Fake Arya spot (they can't show all of it explicitly anyway) and Theon could save her. Also, Melisandre's vision about a girl on a horse could be kept, cutting Alys Karstark.