After leaving the Twins, Arya will likely head south in search of the Brotherhood. She knows they're nearby harassing the Freys, and she probably knows the approximate area of their hideout, where she was held in season three. Since the Hound is with the Brotherhood, this is likely where they'll meet again. When Thoros and Beric sold Gendry to Melisandre, all three of them ended up on Arya's list. Melisandre herself prophesied that she and Arya would meet again. With Melisandre an outcast from Jon Snow's service and exiled from the kingdom of the North, she will need to ride south of the bogs of the Neck in order to avoid execution. This will put her close to the Twins and the Riverlands in the near future. Melisandre may also seek out the Brotherhood, in order to get advice from the only other red priest in Westeros—Thoros of Myr.

Arya will attempt to kill her three Riverlands targets, but I suspect she won't be completely successful. In particular, I think Melisandre still may have a role to play in the war with the White Walkers. Arya may kill Thoros or Dondarrion, but be caught by Melisandre or the rest of the Brotherhood. I think that Thoros is most likely to die, because if she fails to kill him, he can just perform his Lord of Light mojo-jojo and bring back Dondarrion, which would cheapen the kill for viewers. If Arya is captured, the Hound may have to decide where his loyalties lie—with the group of outlaws who once captured him, put him on trial, then stole his money, or with the wild wolf-girl who was his traveling companion and erstwhile sidekick for months. It will be interesting to see how Sandor reacts to seeing the girl who left him for dead, and how Arya reacts to seeing the Hound alive and well.