Just my own bit of speculation, based on the small scraps we have about Hell Bent. I think Oswin could potentially come back, and that this could potentially be her. In that location, we also know that the Doctor is likely to be playing guitar again.

The two of these have made me think of two very crucial things. Why the waitress? And why the guitar?

It reminded me of the little bit of dialogue we've got from the episode, as per the episode synopsis:

“Is it a sad song?”
“Nothing’s sad till it’s over. Then everything is.”
“What’s it called?”
“I think it’s called Clara.”
“Tell me about her."

For theory sake, lets apply the characters to that...

Oswin: “Is it a sad song?”
The Doctor: “Nothing’s sad till it’s over. Then everything is.”
Oswin: “What’s it called?”
The Doctor: “I think it’s called Clara.”
Oswin: “Tell me about her."

If they are bold enough to kill Clara off as I feel might just potentially be the case in Face the Raven (one of three intruders will die...I feel The Doctor and Me will live), then the finale will see Oswin act as consolation.

So back to my questions. Why a waitress? And why the guitar? Well the guitar is explained by the 'sad song' called 'Clara' that the Doctor possibly plays in the diner. The waitress? I think The Doctor might just get a soufflé on the house from a waitress whose handing in her notice for a career change - one that will eventually put her on a path to the asylum. At this point The Doctor has to accept that now having met Oswin pre-Asylum, he can no longer try to save Clara... her death led him to Oswin, in turn Oswin led him to Clara back in Series 7. If he intervenes with her death, he may never have met Oswin, and so would never meet Clara...and he would lose all their precious time together...“Nothing’s sad till it’s over. Then everything is.”