Well, perhaps the Flood from Halo somehow gets mixed up with the zombie virus in the Walking Dead universe. The characters from both worlds would then have to figure out how to stop this new super - virus. They could form alliances between Spartans and some of the more capable survivors from the Walking Dead. For example, Daryl could team up with a Spartan to take on hordes of Flood - zombies. It would be a really chaotic and action - packed scenario.
One idea could be that the Winchester brothers from Supernatural end up in the world of The Walking Dead. They could use their hunting skills to fight off the walkers. Maybe they'd discover that the zombies are somehow related to a demonic presence and try to find a way to stop it.
One idea could be that the Winchesters find themselves in the world of The Walking Dead. They might use their hunting skills to deal with both the zombies and any supernatural elements that start to emerge in that world. For example, they could discover that a witch is causing the zombie apocalypse as part of a dark spell.
One idea could be that Bella, with her unique ability to attract vampires in her original story, has an unexpected reaction to the zombies in The Walking Dead universe. Maybe her scent or some hidden power she has confuses the zombies, leading to some comical or strategic situations for her group.
Well, perhaps the Cullens could form an alliance with a group of survivors from The Walking Dead. They could use their knowledge of the area around Forks and their skills to help each other. For example, the vampires could protect the humans at night when the walkers are most dangerous, and the humans could help the vampires during the day when they are more vulnerable. And what if Bella's special ability as a shield could be used to protect a whole group from the walkers?
A cool 'Walking Dead Crossover Fanfiction' idea is to cross it with 'The X - Files'. Mulder and Scully could be investigating the zombie outbreak. They might find out that it's not just a normal virus but some kind of government experiment gone wrong, which ties in with the conspiracy theories they usually deal with. And the characters from 'The Walking Dead' would be skeptical at first but then realize they need Mulder and Scully's help to figure out the real cause of the zombies.
One idea could be that some of the Sons of Anarchy members find themselves in the world of The Walking Dead. They might use their motorcycle skills to outrun the walkers in the open roads. Maybe they try to form an alliance with Rick's group, bringing their own set of survival skills like fixing vehicles and making weapons from scraps.
A cool 'fear the walking dead crossover fanfiction' idea would be a crossover with 'Stranger Things'. The Upside Down could be connected to the zombie outbreak in some way. The kids from Hawkins could team up with the survivors of 'Fear the Walking Dead' to fight both the zombies and the strange creatures from the Upside Down. It would be interesting to see how their different skills and knowledge would work together.
One idea could be that the Winchesters somehow end up in the world of The Walking Dead. They might use their hunting skills to fight the walkers in a different way than the survivors are used to. Maybe they try to find a supernatural cause behind the zombie apocalypse.
One plot idea could be that Daryl is bitten by a strange creature in The Walking Dead world, and instead of turning into a zombie, he's transported to the Twilight world. There, he discovers that the bite has given him some sort of immunity or power that the vampires and werewolves are interested in. He has to navigate the politics of the Twilight world while also trying to find a way back home. Another idea is that the zombie virus somehow spreads to the Twilight world, and Daryl teams up with the Twilight characters to stop it from taking over. His knowledge of fighting the undead would be crucial in this situation.
One idea could be that the Servants from Fate are transported to the Halo universe. For example, Saber could find herself in a battle against the Covenant. Her noble phantasm could be a powerful weapon against the alien foes. Another plot could involve Master - Spartan team - pairings. A Spartan could be paired with a mage to fight a common enemy that threatens both the Halo and Fate worlds.