A really interesting plot could be that the Sekirei are actually an ancient magical race that was lost in time and has now resurfaced in the Harry Potter world. The Ministry of Magic is initially skeptical but as they see the Sekirei's powers, they realize they could be a great asset. However, there are some dark wizards who want to use the Sekirei for their own evil purposes. Harry and his friends must protect the Sekirei and also use their powers to defeat the dark forces. The Sekirei could introduce new types of magic and relationships to the Harry Potter story, making for a very exciting crossover.
Well, perhaps the Sekirei start to appear in Hogwarts. Some students might accidentally bond with them. Harry could be in a dilemma as he tries to balance the chaos this causes in the school with the need to keep the wizarding world safe from external threats like Voldemort. The Sekirei's abilities could add a new dimension to the magical battles in the Harry Potter universe.
A plot could involve a wizard from the Harry Potter world accidentally stumbling into the Sekirei world. He discovers the Sekirei system and realizes that there are similarities between the magic there and the magic he knows. He then teams up with a Sekirei and they try to find a way back to his world while also getting caught up in the Sekirei battles and politics. Along the way, they learn from each other's cultures and magics, and perhaps even find a way to use the two magics together to overcome various obstacles.
Maybe Gohan is summoned to the Sekirei world for some unknown reason. He then befriends some of the Sekirei and starts to train them in martial arts. This would not only help the Sekirei in their battles but also change their perspective on strength. Gohan's kind heart and strong sense of justice would lead him to oppose the Ashikabi system if he finds it unjust, and he'd try to find a new way for the Sekirei to coexist peacefully.
One possible plot could be that the Straw Hat crew sails to the world of Sekirei. Luffy might be intrigued by the concept of Sekirei battles and try to participate in a friendly way. Maybe Zoro would find some Sekirei with interesting sword - fighting skills and have duels with them. And Nami could be interested in the economic aspects of the Sekirei world, like how they trade resources or manage their finances.
Well, it could be about Shirou getting involved in the Sekirei battles. Maybe he uses his skills from his original story to help or compete in the Sekirei world. For example, his sword - fighting abilities might give him an edge in combat against other Sekirei or their Ashikabi.
Well, perhaps the magical creatures from the Harry Potter universe end up in the '300' setting. Dragons could be used by the Spartans to fight their enemies. The centaurs could also join the battle, lending their archery skills. And Harry could learn some combat skills from the Spartans, which he could then use back in the wizarding world to fight Voldemort's Death Eaters.
One interesting plot twist could be that the magic in the Harry Potter world is actually related to the Sekirei power source. So, when the two worlds meet, they start to influence each other's magic in unexpected ways. For instance, spells start to have different effects when cast near a Sekirei. This forces the characters from both universes to re - evaluate their understanding of magic and power. It also leads to new alliances and conflicts as everyone tries to figure out what is going on.
A plot could center around a lost Star Wars character ending up in the Sekirei world. This character, perhaps a young Jedi in training, has to find a way back home. In the process, they befriend a Sekirei and start to understand their way of life. Meanwhile, back in the Star Wars galaxy, the Sith sense an unusual power shift due to the connection between the two worlds. They send agents to the Sekirei world to investigate and try to disrupt the harmony there. The young Jedi and their Sekirei friends must then work together to stop the Sith and also find a way to reopen the portal to send the Jedi home.
The Stargate could be a portal between different magical realms in the Harry Potter universe. For example, it could lead to a place where ancient and more powerful magic exists. Wizards from different schools around the world could team up with Stargate explorers to uncover the secrets of this new place. There might be a new threat emerging from this realm that requires both magic and Stargate technology to defeat. This would bring together characters like Harry Potter and Jack O'Neill to lead the charge against the common enemy.
Well, perhaps there's a rift in the multiverse that connects Central City to Hogwarts. The Flash, while chasing a villain, gets sucked into the wizarding world. Harry Potter and his friends initially suspect him as a threat but then realize his good intentions. They then combine their powers - the Flash's speed and the magic of the wizards - to fight a common enemy. For example, the Flash could use his speed to distribute protective spells all over Hogwarts really quickly, while Harry and his friends focus on the offensive magic against the bad guys.
One idea could be that Carly and her friends discover a magical portal in their apartment that leads to Hogwarts. They then have to navigate the wizarding world, with Carly being fascinated by all the magic. Meanwhile, Harry Potter and his friends are intrigued by the strange Muggle - like devices the iCarly gang has. They end up teaming up to solve a mystery that involves both the wizarding and Muggle worlds.