Timeline Harry Potter - after Order of the Phoenix. Buffy the Vampire Slayer - after Chosen.
Disclaimer None of these characters belong to me. All Harry Potter related characters, settings etc belong to JK Rowling; all BtVS characters etc belong to Joss Whedon.
Buffy watched the young girl as she left the room, and felt a small pang of sadness for her - she had a destiny now, whether she liked it or not. Of course, she could say no, but ... but Buffy had a feeling that this wasn't an option for this young girl. This young Slayer. Inhaling deeply, she turned to face the others at the table. At least Hermione wouldn't be alone.
"So, Professor," Giles was saying, "we shall come back to Hogwarts in about a week. You said you had a long weekend coming up then?" Dumbledore nodded affirmatively. "Obviously, as we have discussed with her, there is little question of Hermione not continuing her education - she is a bright young woman, and a Slayer who is a witch is something of a rarity."
Buffy had been extremely surprised when, after only a few days in England at Giles' house/mini-mansion, an owl had arrived. Carrying a letter. Addressed to "Rupert Giles and his Slayer." It had requested that the bizarrely-named Albus Dumbledore, Headmaster of Hogwarts School of Witchcraft and Wizardry, be allowed to visit them to discuss matters of great importance. Giles had written back, and the man - resplendent in purple and midnight blue robes with a beard down to his knees and half-moon glasses - had appeared the next day. Out of the fireplace.
To say the least, the assembled Scoobies were shocked, all with the exception of Giles, who professed to have known Dumbledore in his days as a young Watcher, when he had been doing the less than fun job of Watchers' Council - Ministry of Magic liason. According to Giles, Albus Dumbledore had been the only wizard who had made that job bearable in the slightest; the only wizard who believed in firm cooperation between the two bodies.
They had all been even more surprised to learn of Hogwarts School of Witchcraft and Wizardry ("No fair!" Willow had said laughingly). And the fact that Professor Dumbledore thought there may be a Slayer there. So, they had agreed to come and meet the young woman, just Xander, Buffy, Willow and Giles.
Dawn had stayed at 'home' although they didn't have one now. She had been intriuged by the idea of a school of Witchcraft and Wizardry, but Buffy had preferred that Dawn stay in London. She was enjoying it anyway, and had made friends with some of the Slayers, particularly those who had made it through the fight at the Hellmouth.
Once again Buffy turned her attention back to the conversation, hoping no-one had noticed her momentary lapse in concentration. "Who will train the girl?" Xander asked. All faces turned to Buffy.
"Me, I guess," she said slowly. "I mean, there's Faith aswell, so it isn't like the other girls would be stuck for someone to train them, but there are alot of them." She laughed wryly. "And Giles' house - big, but not big enough." She ran her hands through her hair. "There's alot to sort out, y'know? Where to train, how to organise the council and the slayers, where to live, Dawnie ... "
The old man nodded and peered at her through his half-moon glasses. "My dear, I would be glad to assist where ever I can. Rupert was a good friend back in the old days, and I have heard glowing reports of you and all you have accomplished." Dumbledore paused. "There is another matter I would like to discuss with you too," he said slowly, "pertaining to Miss Granger in part. As you may have gleaned from the early part of our meeting, she is close friends with a young man named Harry Potter ..."