All are crossovers with BTVS.
1. One Piece. Zoro has often been called a demon but his crewmates never once thought that it was true. Or rather, that it USED to be true. Zoro is an ex-vengeance demon who is trying to remember how to be human.
2. Yugioh. The Intitative gets a hold of the Dark Magician (AKA Mahaado, former High Priest of the Sennen Ring). The Hellmouth blocks him from returning to the Shadow Realm and Yami can't find him.
3. Yugioh. Back in Ancient Egypt, Atemu always felt that Mahaado wasn't completely human. Or at least, that he hadn't ALWAYS been human.
Mahaado can be a former demon, a half demon, vampire with a soul... He could be like Illyria or he could be a Higher Power. Or he could be something completely different. It's up to you and there are many possibilities.
4. Yugioh. The New Watchers Council comes across information about the Nameless Pharaoh and his faithful, otherworldly, high priest (Mahaado). Everyone is fascinated with the story and the never expected to meet the two of them in person.
5. Ranma 1/2. Dawn runs away to China and falls (either accidentally or deliberately) into Pool of Drowned Boy. She creates a new identity for herself, as a boy called Ryouga. Then later, Ryouga falls into another cursed Spring, preventing 'him' from turning back into a girl.
They're probably not that good, but I'm sure someone can do something with them.