A great plot could involve a portal opening between the two worlds. Harry, Hermione, and Ron accidentally stumble through it. In the Dark Souls world, they find that their magic works differently. They meet characters from Dark Souls who are initially hostile but then realize they have a common enemy in the form of Voldemort's forces that are starting to corrupt the Dark Souls world as well. Harry has to learn the ways of the Dark Souls combat and use his wits and magic to form an alliance. They then return to Hogwarts with new powers and knowledge to face Voldemort in a final battle that combines the magic of Harry Potter and the dark and gritty elements of Dark Souls.
One possible plot could be that Ladybug and Chat Noir are somehow transported to the Dark Souls universe. They find themselves in a desolate and dangerous land, facing powerful enemies they've never encountered before. Ladybug's Lucky Charm ability might work in strange ways, like creating items that have a different use in the Dark Souls world. Chat Noir could use his cat - like agility to navigate the treacherous terrains. Together, they have to find a way back home while also uncovering the secrets of this new and menacing realm.
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 idea could be a crossover where the characters from 'Harry Potter' and 'Beyond Two Souls' meet in a magical realm. For example, Jodie Holmes could use her powers to enter the wizarding world. Maybe she helps Harry Potter solve a mystery that involves a dark force that is connected to both their worlds, like a powerful entity that feeds on fear and uses the unique magic systems of both universes to grow stronger.
One possible plot could be that the Ashen One from Dark Souls is transported to the Marvel Universe. They might be mistaken for a new superhuman or a threat. The Ashen One's skills in combat and their ability to endure great hardships could make them a formidable force. They could team up with the Avengers, perhaps initially clashing due to misunderstandings but then realizing they share the common goal of protecting the world from greater threats like Thanos or other powerful villains. The Ashen One could teach the Marvel heroes about the concept of 'kindling' or 'linking the fire' which could be metaphorically related to the heroes' own struggle to protect their world and its various sources of power.
One interesting concept could be the combination of magic from Harry Potter and the challenging combat and dark atmosphere of Dark Souls. For example, a fanfic might explore how a Hogwarts student would fare in the Dark Souls world, having to rely on both their spells and newfound combat skills. Maybe they'd use the levitation charm to avoid dangerous terrain or use protego against some of the deadly attacks in Dark Souls.
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.
Well, perhaps the Death Note from the Bleach universe gets mixed up with the magic in Harry Potter's world. A powerful wizard could get hold of it and start using it in ways that disrupt the balance of life and death in both worlds. Harry and his friends would have to figure out how to stop this misuse, with the help of some Bleach characters who understand the true nature of the Death Note.
One idea could be that Eragon, while on one of his adventures in Alagaësia, accidentally creates a portal that leads him to Hogwarts. There, he meets Harry Potter and they quickly realize they have a lot in common. They could team up to fight against a dark force that somehow threatens both their worlds, like a new type of dark magic that combines the magic of Alagaësia and the dark arts from the wizarding world.