Perhaps a magical or otherworldly event causes a rift between the two universes, and characters from both Berserk and Vinland Saga start to swap places randomly. Some of the Vikings in Vinland Saga find themselves in the nightmarish world of Berserk, facing demons, while some of the characters from Berserk end up in the more historical and morally complex world of Vinland Saga. This chaos could lead to all sorts of interesting interactions and character developments as they try to find their way back to their respective worlds.
One idea could be that Guts from Berserk ends up in the world of Vinland Saga. He might cross paths with Thorfinn and they could either become allies or initially clash due to their different outlooks. Guts' brutal fighting style and his cursed brand could be a source of mystery and awe for the characters in Vinland Saga.
A possible plot could involve the forces of evil in Berserk somehow making their way into the Vinland Saga world. Thorfinn, with his newfound sense of peace and purpose, has to team up with Guts to fight off these dark forces. They would have to learn from each other's combat techniques and ways of thinking. Guts could teach Thorfinn more about raw, unbridled combat, while Thorfinn could show Guts the value of redemption and a different kind of strength.
Casca from Berserk and Gudrid from Vinland Saga could be an interesting pairing. Casca has been through so much trauma, and Gudrid is a strong - willed woman in her own right. They could bond over their experiences as women in male - dominated worlds, whether it's the world of war in Berserk or the Viking society in Vinland Saga. Gudrid could perhaps help Casca in her recovery in some way.
Another interesting concept is to have the characters from Berserk interact with the magical creatures and races of the kingdom. The Band of the Hawk could be hired as mercenaries by the kingdom to fight off an invasion of some sort. This would lead to some great action scenes and also allow for character development as they interact with the new and different people of the kingdom.
One idea could be a Berserk and Dark Souls crossover. Picture Guts in the dark and unforgiving world of Dark Souls, facing off against the powerful bosses there with his greatsword. His tenacity and fighting skills would make for an exciting addition to the Dark Souls lore.
One idea could be exploring Gwain's backstory before the events of the saga. Maybe his family life, his early training, and how those experiences shaped him into the character we know in the saga.
Another option is a crossover with the Pokémon world. The Phantom Thieves could travel to a region in the Pokémon world and discover that someone is using Persona powers to make Pokémon act out of control. They would need to catch Pokémon themselves and use their combined powers of Personas and Pokémon to stop the bad guy. It would be interesting to see how they interact with Pokémon trainers and the whole Pokémon ecosystem.
A cool plot could involve the Persona 5 crew going back in time to the events of Persona 3. They might end up interfering with some of the original events, which could have unforeseen consequences. For example, they could accidentally change the way a key character in Persona 3 awakens their Persona, and then have to figure out how to fix it while also dealing with the Shadows in that era.
One idea could be to cross it with another popular RPG. For example, having the Persona 4 characters enter the world of Final Fantasy. They could interact with the different races and magic systems there. Another could be a crossover with a detective - themed anime. The Investigation Team could team up with the detectives from that anime to solve a mystery that spans both worlds.
Another plot could involve a crossover with the Punisher and the X - Men. Frank Castle might find himself in a situation where he has to work with mutants. He could initially be distrustful but then realize they are fighting for a similar cause against a common enemy that is targeting both humans and mutants. For example, a group of anti - mutant and anti - vigilante fanatics.
One idea could be a crossover with Peter Pan. Tinkerbell could meet characters from another world, like Alice from Wonderland. She might be confused by the strange rules and creatures there, and her magic could interact in unexpected ways with the Wonderland magic.
Another plot could involve a crossover with a post - apocalyptic world. The Boys find themselves in a world where society has collapsed, and they have to team up with the survivors against new threats. This could bring out new sides of the characters as they are forced to adapt to a completely different environment and face challenges they've never faced before.