Maybe the butler is a time - traveler. He comes from a future where Voldemort has won and he tries to change the course of history by helping Harry. He could introduce new spells or magical devices from the future to Harry, giving him an edge in the fight against the Dark Lord.
The butler could be a house - elf who has been freed but still feels a sense of loyalty to the Potter family. He serves Harry not just as a butler but also as a guide to the secrets of Hogwarts and the wizarding world. He might know hidden passages and forgotten spells that could help Harry in his adventures.
One possible plot could be that the butler is actually a wizard in disguise who has been sent to protect Harry. He could use his knowledge of the wizarding and Muggle worlds to keep Harry safe from both magical threats like Death Eaters and Muggle dangers. For example, he could use his connections in the Muggle world to find safe houses for Harry when he needs to hide from Voldemort.
One idea could be that the butler has a secret magical ability that he uses to protect Harry from hidden threats in the wizarding world. Maybe he can turn invisible at will and spy on Harry's enemies.
One idea could be that the demon butler is secretly protecting Harry from a new, unknown dark force in Hogwarts. He uses his demonic powers to sense danger before it arrives.
Perhaps Sebastian Michaelis could be summoned by a Hogwarts student who is dabbling in dark magic. He appears in the wizarding world and causes chaos. Harry and his friends suspect something is wrong and start to investigate. Along the way, they discover the existence of demons in a different context than they thought, and they have to use both magic and their wits to deal with Sebastian and the problems he brings.
Dobby could be a great butler. He was always eager to serve Harry and was very loyal. His knowledge of the Malfoy manor and other wizarding households could be useful. And his unique brand of magic, like his ability to apparate in and out of places where other wizards couldn't, would add an interesting dynamic to the story.
Maybe a dark wizard in the Harry Potter universe has found a way to access the power of the One Ring from Middle - earth. Aragorn, sensing the danger, travels to the wizarding world with the help of some elvish magic. He teams up with Harry Potter to stop the dark wizard from using the Ring's power. Along the way, they face many challenges, including dealing with the corrupting influence of the Ring on those in the wizarding world who are power - hungry.
Well, the plot could involve the cat - Harry Potter - stumbling into the Black Butler world. Maybe he's lost his human form due to some dark magic. He could then encounter Sebastian and Ciel. The cat - Harry - might use his magic in cat form to help or hinder the duo in their investigations. For example, he could use his invisibility ability (in cat form) to spy on enemies.
One implication could be the confusion of the two fan bases. Fans of Harry Potter might be put off by seeing their beloved characters in a context that doesn't align with the original series. Similarly, Black Butler fans may not appreciate the distortion of their favorite story.
One interesting crossover could be the characters from Black Butler ending up at Hogwarts. Maybe Sebastian could be disguised as a new professor, teaching a class on dark magic in a very unique way. His butler skills would surely make him stand out among the Hogwarts staff.
The plot could involve Harry trying to hide his new wings at first. He might be afraid of how the Avengers would react. But then, as the story progresses, his wings could become an asset in a big battle against a common enemy. Maybe the enemy has some sort of anti - flight technology, and Harry's magically - enhanced wings are the key to victory.
One interesting Harry Potter butler fanfiction could be about a magical butler who secretly protects Harry from the shadows. This butler might have special powers and knowledge that help Harry in his adventures.