In a more comical take, Ron Weasley could end up in the DxD world and mistake a Devil for a new kind of magical creature. He tries to befriend it using his typical Ron - clumsy but endearing - ways. Meanwhile, Hermione is trying to figure out the scientific and magical laws that govern the DxD world. They all get involved in a school - tournament in the DxD world that combines magic and combat skills, much like the Triwizard Tournament in Harry Potter.
One idea could be that Harry Potter, while exploring some new magic, accidentally opens a portal to the DxD world. He then has to adapt to the new rules and powers there. Maybe he teams up with Issei and they use a combination of Hogwarts magic and Sacred Gears to fight off a common enemy.
Perhaps Voldemort somehow makes his way into the DxD universe and starts causing chaos. The Devil Kings in the DxD world are initially confused by his dark magic but then they join forces with Harry and his friends. Harry could teach them some unique spells from Hogwarts, like the Patronus charm, which could be useful against Voldemort's Dementors - like minions he might create in the DxD world.
A really cool plot could be that the Deathly Hallows from Harry Potter have some kind of connection or effect in the DxD universe. Say, the Elder Wand gives the wielder an extra boost of power in DxD battles, but it also attracts unwanted attention. And Harry, being the owner, has to navigate through the complex politics and battles in the DxD world while protecting the wand. Also, the resurrection stone could bring back some important characters from DxD's past, causing all sorts of chaos and new storylines.
A plot could involve a threat that endangers both universes. So, Harry Potter and his friends team up with Issei and his group. They start by trying to understand each other's worlds and powers. As they do, they find that their combined magic and skills are the key to defeating the common foe. This could involve things like the Hogwarts students teaching the Highschool DxD characters about complex spells, while the Highschool DxD characters share their combat techniques and unique powers.
One plot idea could be the characters from Harry Potter learning about the different types of power and ranking systems in DxD. For example, Harry might be intrigued by the concept of Sacred Gears and try to figure out if there's anything similar in the wizarding world.
One idea could be that Harry Potter somehow gets transported to the world of Highschool DxD. He could use his magic skills in the unique battles and challenges there. For example, he might use spells like Expelliarmus to disarm the devils or fallen angels during a fight.
One possible plot could be that Harry Potter, while on a new magical adventure, somehow gets transported to the world of Highschool DxD. He could use his unique magic skills to help the characters in Highschool DxD deal with their own supernatural threats. For example, his Patronus charm might be effective against some of the evil creatures in the DxD world.
Well, there could be a story where a powerful dark magic object from the wizarding world is brought to Sunnydale. Buffy discovers it and realizes it has the power to disrupt the balance between good and evil in her world. She then travels to Hogwarts to seek help from Harry Potter. Along the way, she meets various magical creatures and has to navigate the wizarding world's politics. Once she meets Harry, they discover that there is a common enemy behind all this, someone who wants to use the chaos in both worlds for their own gain, and they must work together to stop this mastermind.
One idea could be a time - travel plot where a character from Legacies goes back in time to the era of Harry Potter. They could interact with the young Harry and his friends, and perhaps help in the fight against Voldemort in unexpected ways. For example, they might introduce a new spell from the future that gives the Order of the Phoenix an edge.
One idea could be that the Grimm creatures start to appear in the wizarding world. Harry and his friends have to use both magic and knowledge of Grimm lore to fight them. For example, they might discover that some dark magic has opened a portal between the two universes.
One idea could be that the Order of the Phoenix teams up with the Kingsman. The wizards could use some of the Kingsman's high - tech gadgets and fighting skills, while the Kingsman could learn about magic. For example, Harry Potter could be trained by a Kingsman agent in hand - to - hand combat, which would give him an edge in battles against Death Eaters. Another plot could involve a villain who is a hybrid of a dark wizard and a criminal mastermind that the Kingsman usually deal with. The two groups would have to combine their resources to stop this new threat.
Maybe some of the Hogwarts students are sent to the Naruto universe. They would be amazed by the ninja techniques. Hermione, for instance, would be eager to study how chakra works and might even find a way to combine it with magic. Harry could learn some ninja stealth skills which would be useful in his fight against Voldemort.