A/N Anyanka has been around for over a thousand years. And Sunnydale for over a century. Surely she's visited before. Willow was awfully upset after finding Xander and Cordelia kissing in the stacks that day. What if it was enough to summon Anyanka? What would Willow wish for and what would the results be? Like any wish, simultaneously, what she asks for, but not what she is expecting.
I don't think Anyanka was powerful enough to actually create alternative realities. More likely the wishes she granted could alter a key moment in history and a new timeline would occur, the original timeline just continuing on from that point. Or she moved the wisher or wishee to a reality where circumstances matched the stated wish. Unless the wish involved boils or penises falling off or something. Than she'd just make that happen. Which means there is a reality were Xander and Willow are vampires and she moved Cordelia there after altering the moment of Buffy going there, maybe. Scary enough. So for purposes of this fic, Anyanka will be moving our participants to other realities were events occurred that match the stated wish.
Willow wandered out to the courtyard of Sunnydale High, feeling miserable. She had just caught Xander, her Xander! kissing that skanky ho, Cordelia of all people. How could he do that to her? Ok, granted he had never shown any interest in her, Willow, but to kiss Cordelia!
Willow wondered what, beside her tits, Xander could see in her. It just didn't make any sense. Cordelia was mean and bitchy and she would probably dump him in a week anyway, so why was Xander even messing around with her. Was she putting out? Scary thought as Willow had always thought she and Xander would be each other's firsts. That he might not have saved himself for her was as painful as the idea of him with Cordelia. But what could she do about it?
Someone sat down beside her on the bench and said. "You look like I feel. Just yesterday I found out my boyfriend has been cheating on me, I just want to wring his neck. I mean how dare he, you know? Cheat on me?!"
"I just found the guy I've been in love with for years, my best friend, kissing the bitch that has made my life hell for years."
"Men can be so stupid, don't they recognize a good thing when they see it? It just makes me want to, to, I don't know, to wish something would happen to them."
"If I could wish something, I think I would wish that me and my friends and that skank ho he was kissing to get the life we want, the one that makes each of us the happiest. That would keep her far away from him!" Willow looked over at the other girl and was startled to see her face had changed. She now looked like her veins and muscles were somehow weirdly exposed. Her necklace was glowing.
In a gravely voice she said "Wish granted!" A moment later and the courtyard was empty as Willow Rosenberg disappeared. Along with Buffy Summers, Alexander Harris, Cordelia Chase and the vampire who had just recently lost his soul.