Perhaps there could be a character in the Harry Potter world who has vampire - like abilities and is mistaken for a real vampire. D is called in to deal with this 'vampire' but realizes it's not a normal vampire case. He and Harry then have to figure out what's really going on. It could involve a dark wizard's experiment gone wrong that created this half - vampire - half - wizard creature, and they need to find a way to reverse it or at least keep it from causing harm.
One idea could be to cross Vampire Hunter D with a post - apocalyptic world setting from another series. D could be on a quest to find a vampire lord who has the key to a cure for a virus that has decimated humanity. He would face both vampires and mutated creatures along the way.
The clash of different power systems would be really interesting. D has his superhuman strength, speed, and the ability to use his left hand for various powers, while the wizards in Harry Potter have magic. How would they interact? For example, would D's powers work against magical creatures in the wizarding world? And how would the wizards react to D's unique abilities? It could create some really cool battle scenes and moments of discovery.
One idea could be that Harry Potter discovers a hidden world within the wizarding realm where Vampire Hunter D's technology and lore exist. He teams up with D to fight a new breed of vampires that are immune to normal wizarding spells. This would blend the magic of Harry Potter with the dark, post - apocalyptic feel of Vampire Hunter D.
A crossover with "Castlevania" would also be cool. D could find himself in the world of Castlevania, dealing with the various dark creatures there. He might have to navigate the complex politics between the vampires and the humans in that universe.
Another option could be Van Helsing from the movie of the same name. He is a classic vampire hunter. He and D could exchange hunting techniques and knowledge. And they could form an alliance when they discover a group of vampires that are planning something really big, like taking over a major city.
One idea could be that Megara, with her sharp wit and street - smart nature, finds herself in the wizarding world of Harry Potter. She could team up with Harry and his friends, using her own unique skills to help them out in a dangerous situation, like facing a new dark wizard threat.
They could interact through a magical artifact. For example, a magical amulet in the Harry Potter world might have the power to summon Vampire Hunter D. When Harry finds and activates it, D appears. D is initially distrustful of Harry's magic but soon realizes they can work together against a common enemy - the vampires.
One idea could be that in the 'Harry Potter' world, a new magical game is introduced which is similar to 'Monster Gamer'. The characters from 'Harry Potter' start playing it and discover that it has some real - world magical implications. For example, the monsters in the game might start appearing in Hogwarts and the students have to use their magical skills to defeat them, just like in the game.
One idea could be a crossover with the Lord of the Rings. Imagine Harry and his friends traveling to Middle - earth to help in the fight against Sauron. They could use their magic in new and unexpected ways in that different magical realm.
One idea could be to have Harry Potter characters enter the world of Final Fantasy VII. For example, Harry could end up in Midgar and meet Cloud. They could team up to fight against the dark forces in both their worlds, like Voldemort's minions and Sephiroth. The magic from the Harry Potter universe could interact in interesting ways with the materia and summons of Final Fantasy VII.