Gendry Theory
I think he has to reappear. It seems pointless to show him being sent off, never to return to the show in a not-too-satisfying conclusion.
With time running out and the plots getting narrower, I've wondered what his role could be. I just had a thought that maybe he never made it off of Dragonstone and he's been hiding out somewhere on the island all along. Then, maybe he gets involved in forging weapons out of Dragonglass. This would make use of his skill and give his character more of a purpose for returning to the show. The fact that he's Robert Baratheon's son doesn't need to even come up again (even if Dany found out, I think she could overlook it given the circumstances). It would be nice if he had a reunion with Arya though.