One possible plot could be that Wolverine somehow ends up in the Twilight universe. Maybe he's on a mission that goes wrong and he gets transported there. He then meets the vampires and werewolves in Twilight. The vampires might be intrigued by his healing factor and his adamantium claws. And the werewolves could see him as either a threat or an ally. He could get caught in the middle of the conflict between the Cullens and the Volturi, using his combat skills to tip the balance.
Well, perhaps Wolverine is sent to the Twilight world to hunt down a mutant - like creature that the government thinks is there. Once there, he crosses paths with Bella. She's fascinated by his rough exterior and his mysterious past. He starts to question the nature of the vampires and werewolves in this new world. The Cullens initially view him with suspicion but then realize he could be an asset against common enemies. They team up and Wolverine teaches them some new combat techniques using his claws and berserker rage.
One possible plot could be that Wolverine somehow gets transported to the wizarding world. He could be mistaken for a dark creature at first because of his claws and rough appearance. But then he teams up with Harry Potter to fight against a new dark wizard who has the power to control metal, which poses a great threat to both the Muggle and wizarding worlds. Wolverine's healing factor and combat skills, combined with Harry's magic, would make for an exciting adventure.
Maybe the plot could involve a new type of mutagenic weapon that is being developed in Gotham. Wolverine is sent to investigate as it could pose a threat to mutants. He meets Batman who is also on the case as it could be used by Gotham's criminals. They start off distrusting each other. Wolverine doesn't trust this masked vigilante, and Batman is wary of Wolverine's wild nature. However, as they dig deeper, they find that a common enemy from their pasts is behind it all and they must combine their skills to stop him.
One possible plot could be that the vampires from Twilight somehow end up in the world of Glee. Maybe they are drawn there because of a unique musical energy. The Cullens could enroll in McKinley High and join the Glee club. Edward, with his musical talent, could be a star singer, but his vampire nature would cause some interesting conflicts, like how to handle his super strength during dance numbers or when handling musical instruments without breaking them.
The plot could involve Naruto being sent to the X - Men's universe. There, he meets Wolverine and for some reason, they have to exchange powers temporarily. Naruto then uses Wolverine's powers in the midst of a big battle against a common enemy, like Magneto who is trying to take over both the ninja world and the mutant world. Naruto's ninja skills combined with Wolverine's brute strength would make for an interesting fight.
One idea could be a Wolverine and Batman crossover. Wolverine could find himself in Gotham City and team up with Batman to take on a common enemy like Ra's al Ghul who is interested in Wolverine's healing factor. They would have to combine their skills, with Wolverine's brute strength and healing and Batman's detective skills and gadgets.
Perhaps Bella could be a key figure in a crossover. She could be a 'dragon rider' in a sense, using her connection with Edward (similar to the bond between a dragon and its rider in Game of Thrones). Her immunity to vampire powers could translate to some sort of immunity to the dark magic in Westeros. The werewolves from Twilight could be allies with the Stark direwolves, and together they fight against common enemies, like an army of the dead led by a Volturi - like figure.
One possible plot could be that the Volturi from Twilight discover the existence of demigods from the Percy Jackson world. They see the demigods as a threat to their vampire supremacy. Percy and his friends have to team up with some of the more friendly vampires, like the Cullens, to fend off the Volturi. This would bring together the magic and powers of both universes in an epic battle.
A possible plot is that the Capitol in the Hunger Games discovers the existence of vampires from Twilight. They see them as a new form of entertainment and try to capture them to put in the arena. However, the vampires are not so easily controlled. Katniss and her allies could team up with the vampires against the Capitol. This would bring together the themes of rebellion from Hunger Games and the supernatural elements of Twilight.
Well, perhaps in this 'immortal ichigo twilight crossover fanfiction', Ichigo arrives in the Twilight world during a time of great unrest. The Volturi are on the verge of a major power play. Ichigo's immortality gives him an edge in this dangerous new world. He teams up with Bella and Edward. Ichigo's combat skills and reiatsu powers could be a huge asset against the Volturi's army. They could form an unlikely alliance to protect Forks and the rest of the world from the Volturi's tyranny.
One possible plot could be that Inuyasha somehow gets transported to the Twilight world. He might initially be confused by the modern settings and the presence of vampires and werewolves. Bella could mistake him for a new type of supernatural being. And Inuyasha, with his sword and combat skills, could get involved in the ongoing feud between the Cullens and the werewolves in Forks.