I went to see
The Nutcracker yesterday. It was oodles of fun. For several years when I was a child, my family went to see it every year, but at this point, I hadn't seen it for quite some time.
In this particular performance, whenever Drosselmeyer was on the stage, he completely dominated it: he had a great deal of presence. He's a rather dark and mysterious figure with an eyepatch but it also a source of presents and the joy of the party when he's there. He gifts the Stahlbaum parents with two life-size mechanical dolls that dance, and then gives the children, Clara and Fitz, a nutcracker doll. This is the Nutcracker of the title who fights the evil Mouse King on Clara's behalf.
The story is set in Germany in the 1890s.
So my thought is that Xander of the Buffy universe somehow gets thrown back in time. A common enough fanfic plot devise. He was no doubt fighting some evil demon in Germany in our time when it all went wrong. So there he is some hundred years in the past. Luckily, the Watchers are an ancient, international organization with more than a bit of experience dealing with the weird. So he contacts the local Watchers who are unfortunately not able to fix the situation but are able to set him up with a decent enough life.
His own experience (canonical) had given him a lot of anger for the Watchers, but in the past he discovers that there were a few things he had gotten wrong. They did not remove the potential Slayers from their families, and in this time and culture that was actually a problem: the young women were being raised to be completely passive damsels in distress. Something needed to happen. A simple mystical destiny doesn't change that much about a person.
So, he ingratiates himself to the parents of the most likely next potential and finds a way to be made Godfather to the potential-Slayer Clara. Knowing that someone (the Mouse King probably) is out to get her, he gives her a magical guardian in the guise of a Nutcracker.
He realizes how important it was for her to have someone else when she can't even protect her new gift from an annoying little brother.
Anyway, there's the fight and Clara comes through in the end to rescue her Nutcracker Prince.
After the fight is over, when Clara and the Nutcracker Prince are being entertained in the Sugar Plum Court, many of the performances are most likely martial in nature.
The next morning she probably girds her loins and asks to talk with her Godfather Drosselmeyer. He explains a few things to her and she goes on to become one of the more successful Slayers.
Merry Christmas to everyone except the Vampires and Demons.