I think Sandor is on a revenge path that will ultimately lead to an unhappy ending if he doesn’t change course.
Sandor passes along his nihilistic life philosophy to Arya during their time together, and I think Arya adopts his philosophy after the trauma of the Red Wedding, when she is at her most vulnerable moment and the Hound’s philosophy seems to be the most true.
I think Arya’s inability to kill Sandor will haunt her with the knowledge that she isn’t the monster she’s pretending to be. Arya’s mercy toward Sandor stands in contradiction with her ruthless persona. I think she will have to contend with it or else it will characterize her ending.
The gravedigger certainly isn’t the end of Sandor’s story. But I have no idea where his story is going next.