The plot could start with Harry having strange dreams about Shinigami. As he investigates, he discovers that there is a hidden portal in Hogwarts that leads to a realm where Shinigami exist. He and his friends enter this realm, and they find out that there is a dark force in the wizarding world that is related to the Shinigami in some way. They then have to work with some friendly Shinigami to stop this dark force. Along the way, they learn about new forms of magic and the true nature of death from the Shinigami.
One way could be to give Harry the Geass power. So, he could use it to control the actions of Death Eaters or Voldemort himself. He might start to make more strategic and calculated moves like Lelouch, using his new power to outwit his enemies. Another aspect could be in the political side. Maybe the wizarding world becomes more like Lelouch's world with different factions vying for power and Harry has to play a political game as well as fight against the dark forces.
The plot could start with a spell gone wrong in Hogwarts that somehow transports a group of Transformers to the wizarding world. The wizards are initially scared but soon realize that they need to cooperate with the Transformers to deal with a new dark force that is attracted to the combination of magic and technology. The story could involve battles, magic - tech hybrids, and new friendships between the two groups.
One way could be to have Harry be adopted by a Mandalorian family. He grows up learning both magic and Mandalorian combat skills. Then, he has to use his combined knowledge to fight against a common enemy that threatens both the wizarding world and Mandalore.
One way it could work is if there was a magical portal that connected the wizarding world to Spider - Man's universe. Harry could accidentally stumble through it and then get involved in Spider - Man's battles, using his magic to help out.
One way could be through a plot where Harry discovers some long - hidden secrets about Dumbledore that he feels were manipulative. For example, if he found out that Dumbledore had been using him more than protecting him all along. Harry might then confront Dumbledore publicly in the wizarding world, shaming him for his actions.
First off, Harry Potter would be quite disoriented in the Shinigami world. However, his inquisitive nature would drive him to explore. He'd likely start by observing the Shinigami at work. He could draw on his knowledge of the wizarding world's laws and ethics to understand the Shinigami's code. His ability to think on his feet, which he developed during his battles against the Death Eaters, would be useful when he has to make quick decisions in the Shinigami world. For example, when deciding whether to purify a Hollow or not.
One way it could work is if Jack Sparrow somehow ends up at Hogwarts. Maybe he's chasing a magical treasure that has been hidden there. His pirate ways would clash with the strict Hogwarts rules, and Harry Potter and his friends could either befriend him or be suspicious of him. Another possibility is that Harry Potter, being a fan of Jack Sparrow, tries to model some of his actions after the pirate, much to the dismay of his teachers.
One way could be that Godzilla somehow appears in the wizarding world. Maybe a magical experiment goes wrong and transports Godzilla there. Then the wizards and witches have to figure out how to stop it using magic.
Well, one way could be through a magical accident. Maybe a spell gone wrong during a battle or some kind of experiment that binds their magic in an unexpected way, leading to the creation of a child. It could also be a result of a forced situation by a dark force that wants to see the two rival families united in a very strange way.
To develop it, you could introduce a new character who is a hybrid of a wizard and a Cybertronian. This character could be the link between the two worlds. Harry could then be drawn into the Cybertronian world through this character. In the Cybertronian world, Harry could discover that magic has a different manifestation. He could learn to combine his magic with the technology of the Cybertronians. For example, he could use his wand to control the energy flow in a Cybertronian's body, or use a spell to enhance a Cybertronian's weapon. The story could also explore the ethical implications of mixing magic and technology, and how it affects both the wizarding world and the Cybertronian society.