When I was rereading Up, Down, and Strange I was reminded that I had no clue how AU!Buffy became a "ridiculously young watcher." So I decided just now that I'd sketch out my vaguely imagined but never written backstory, which--as I articulate it for the first time--may or may not hang together.
So, we're back in S1 of BtVS. Buffy has transferred to high school on the Hellmouth, a new blonde student who catches the attention of Willow and Xander, though of course in this version, Xander is gaygaygay (though he doesn't realize it yet) so he is only interested in a friendly way, and because she's all mysterious and drops a stake in the hallway.
We follow Buffy and Giles--also a watcher, who has installed himself conveniently as librarian--as they wander the town's graveyards at night, looking for their target: a newly called slayer. Buffy's dad at one point had a diplomatic appointment in England, and Buffy was noticed by watchers and tapped at an absurdly young age--like, six--by the Council as having aptitude. The Council in this universe is always keeping their eyes open for a particular type of watcher, a young girl who will be assigned to work with vampire slayers, actual vampires, to tame them the way a virgin tames a unicorn. (Whatever.) Her parents were confused at first, then proud, and allowed her to be trained. Giles, her mentor, came with her back to America when a new slayer was called.
So now they track down Spike--no Dru with him at this time--and he's called. He hasn't got a soul, but The Powers That Be perpetrate some cosmic mojo on him, and though he resists for a bit in bewildered furious resentment, he has always been a bit of a rebel oddball and he eventually capitulates. If I recall, I also had the idea that they'd collared him to help with compliance, but the details of that remain blurry to me.
Willow and Xander of course stumble across all this, and Scobby hijinks ensue. Let's imagine there's a tunnel between the school basement and Spike's closer crypt.
Angel ("Willow") hits Sunnydale at some early point, a uniquely souled vampire, and Spike's sire. Spike is all, "You might have a soul, Peaches, but I'm the big vampire slayer, nyah nyah." And Angel's like, "I'm here to help you, stupid, and if you don't shut up and behave I'm going to slap you upside the head."
During the first few seasons, Xander falls hard for Spike, but it takes a while for them to hook up because he really has issues with being gay (in the same way that Angel in canon has issues with his demon nature). Eventually he and Spike have a rocky relationship which goes gradually off the rails in late season three. As season three ends, Xander moves to L.A.
Meanwhile, Willow has also been smitten with Spike and has spent a few years trying to lure him away from Xander. This contributed to a rift, but so did her growing magic skills, which went out of control much sooner than in canon. Dark Willow rose in late season-two maybe, in a kind of variation on the Angelus arc (even though she's not "Angel"),then "came back" to herself. However, the Council of Watchers prevented her from ever using magic again, binding her powers. Even though she was all penitent after her recovery, this still grates on her.
Dawn arrives in her key-like fashion in season five; after Buffy's mother dies, Dawn spirals down into this whole juvie phase, acting out the worst type of teen behavior, driven by existential angst and survivor grief. Buffy can't control her and eventually the Harrises foster her.
At some point Dawn and Tara--proprietor of the magic shop--meet and lesbian hijinks ensue. In the wake of Xander's departure, Willow and Spike hook up at last.
Buffy, like Giles, leaves Sunnydale, and later returns. Spike angsts a bit at the abandonment, even though Giles--part of the watcher tag-team--sticks around.
Anya ("Dawn") pops in as she did in canon, a demon-turned-human, and sticks around as a supporting character. I'm not really clear what she does.
I think that's everything.