I also wonder if the hybrid has something to do with Missy
Not quite in the way I was expecting.
I loved that episode, so many great details.
At first I thought the diner with Clara was just another framing device and thought it was unnecessary, but then the episode cleverly gave it context and it was such a sad ending. It was interesting that Clara's theme song got put "in universe" so to speak.
While I thought Clara had a good sendoff for her arc, bringing her back for this episode rounded it out even better by showing not only the bad influence The Doctor had on her, but the bad influence she had on him, with him killing (and having the white dude regenerate into a black chick was a nice choice).
The stock TARDIS was a nice throwback, and I believe Ashildr made a subtle nod to McGann's movie with her question implying he was part human.
The most interesting part was the twist on the Donna Noble ending, with The Doctor forgetting Clara rather than the other way around. It's almost even sadder, because we have to see him live with that, and they made it clear that he'd forgotten nothing else about his adventures. The feels when he saw the mural on the TARDIS, and then it flaked away when he dematerialized.
I was wondering beforehand if the mural would disappear when he dematerialized, and that was before knowing how extra sad that moment would be.
While I instantly thought of The Impossible Astronaut at the start in the Nevada desert, I can't believe I didn't recognize the diner until the end when it did the reverse shot, not long before he realized he'd been there. Silly me.
I'm sure some people (especially Moffat haters) will bitch that Clara actually survived somehow, but I thought it was a fun ending to have Ashildr and Clara in their own TARDIS coincidentally stuck as a diner.
Totally not what I was expecting from that episode, but again a fantastic episode. The Christmas episode however, looks like ass. I hate Christmas episodes, and I hate River Song, so naturally any combination of the two is going to be awful.