Maybe the Sorting Hat could be used on the Transformers. The Autobots are sorted into Gryffindor for their bravery and the Decepticons into Slytherin for their cunning. Then a battle ensues within Hogwarts between the two factions, with the wizards caught in the middle, trying to keep the peace and protect their school.
One possible plot could be that Harry Potter, through some magical mishap or powerful spell, is transformed into a bride. Maybe he has to go on a quest to find a way to reverse the spell while dealing with the strange reactions of his friends and the wizarding world. For example, Ron might be completely confused and Hermione would be trying to find a solution in the library.
Maybe Harry has faced so many traumas and disappointments in the wizarding world that he just decides to distance himself from all the chaos. For example, he might be tired of the constant battles against the Dark Arts and the politics within Hogwarts and the Ministry. He could stop caring about his reputation as 'the Chosen One' and simply want to live a quiet life, away from the spotlight and the expectations.
One possible plot could be that Sesshomaru somehow ends up in the wizarding world of Harry Potter. Maybe a magical portal or a powerful spell goes wrong. He then has to navigate this new world with its own set of rules and magic, and he might encounter Harry and his friends.
A plot might be about Harry's child struggling with the pressure of living up to their father's legacy. At Hogwarts, they constantly feel compared to Harry. But then, they find their own niche in the wizarding world by being really good at potion - making. They enter a big potion - making competition and end up using their skills to save the day when a dark potion goes wrong and starts affecting students. They prove that they can be great in their own way, not just because they are Harry Potter's child.
One plot could be that Harry Potter is bitten by a vampire during one of his adventures outside Hogwarts. He then has to struggle with his new vampiric nature while still trying to protect the wizarding world from dark forces. Another idea is that there is a hidden vampire community within the wizarding world that Harry discovers, and he has to deal with the politics and secrets there.
Well, perhaps Harry discovers some secrets about his family that make him think the Death Eaters' ideology isn't all bad. He could start to secretly interact with some of the less extreme Death Eaters, learning their ways and starting to form a strange sort of admiration for them. But this would be a very different take from the original story where Harry is firmly on the side of good.
A plot could be where Tracey Davis is wrongly accused of stealing an important magical artifact from the school. Harry believes in her innocence and sets out to prove it. Along the way, they find out that there's a much larger conspiracy at play involving some of the other Slytherin students and a group of dark wizards. As they dig deeper, their friendship grows stronger as they face numerous challenges and dangerous situations.
One possible plot could be that some Hogwarts students go on an exchange program to a magical ranch in the American West. There, they meet cowgirls who are also witches. Together, they face a new dark magic threat that is affecting the ranch's magic. The transformation (TF) aspect could be that the Hogwarts students learn new spells from the cowgirls that transform the local magical creatures in unexpected ways.
Perhaps there's a prophecy in both universes that foretells of a coming disaster that can only be averted by the combined forces of the wizarding world and Riddick. Riddick is drawn to the wizarding world because of some strange magic that he senses. Once there, he meets Harry Potter. At first, they don't trust each other. But as they face challenges together, like strange magical - mechanical hybrids created by a rogue wizard who has been experimenting with technology from Riddick's world, they start to respect and rely on each other, ultimately working together to save both universes.
Another interesting aspect is the cultural exchange. In some fanfics, the Autobots or Decepticons try to understand the concept of houses in Hogwarts. They might get sorted into houses based on their characteristics, like the Autobots being in Gryffindor for their bravery. This exploration of different cultures from the two universes is quite engaging.