Perhaps Voldemort somehow manages to bring the Left 4 Dead zombies into the wizarding world. The Order of the Phoenix and the Death Eaters are forced to team up to survive. The Death Eaters' dark magic might be used in new and unexpected ways against the zombies, and the Order's more defensive and protective spells would also be crucial. Harry, being the hero, could figure out a way to stop Voldemort from bringing in more zombies and find a way to send the existing ones back.
One possible plot could be that a powerful dark magic spell goes wrong in the Harry Potter world and creates a portal to the world of The Walking Dead. Harry, Hermione, and Ron find themselves in this post - apocalyptic world filled with walkers. They have to use their magic skills to survive, like using the Stunning Spell to immobilize walkers and the Fire - Making Charm to keep the walkers at bay at night. Meanwhile, some of the survivors from The Walking Dead world are in awe of their magic and start to learn from them, creating a unique bond between the two groups.
One possible plot could be that a spell gone wrong in the wizarding world accidentally opens a portal to the world of The Walking Dead. Harry Potter and his friends find themselves in this post - apocalyptic world. They use their magic to fight off the walkers, but also face moral dilemmas as they interact with the survivors who have no concept of magic. Hermione might try to find a scientific explanation for the walkers while Ron is more focused on survival and getting back home.
Harry could also be very interesting. His experience in facing dangerous foes in the wizarding world would give him an edge. He has a natural courage that would serve him well in the zombie - filled world of Left 4 Dead. His ability to think on his feet and his connection to the wizarding world's resources could make for some exciting storylines. For example, he could use the Marauder's Map to track the movement of the zombies or call on his Patronus to fend them off in a really dark and scary situation.
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 with a post - apocalyptic zombie series. The characters from Left for Dead could find themselves in a different zombie - infested world and have to adapt to new threats and survivors. For example, they might encounter a group that has a very different approach to survival, like a religious cult that believes the zombies are a form of divine punishment. This could lead to conflicts and alliances as the Left for Dead characters try to find their way back to their own world.
Well, it could be about the survivors from Left 4 Dead finding themselves in a world overrun by Nazi zombies. They might have to use their skills and weapons from their own world to fight these new and more menacing foes. Maybe they team up with some resistance fighters who have been battling the Nazi zombies for a while.
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.