A crossover with the Steven Universe universe could also be cool. The Crystal Gems arrive in Gravity Falls during Weirdmageddon. Garnet's future vision could be a great asset in predicting Bill Cipher's moves. Steven's positive attitude and ability to heal could also play a role in bringing hope to the situation. Maybe the gems' magic can interact in interesting ways with the strange magic in Gravity Falls.
One idea could be a crossover with the Gravity Falls and Rick and Morty universes. In this fanfiction, Rick could end up in Weirdmageddon and try to outwit Bill Cipher using his advanced technology and cynical attitude. Morty might befriend some of the Gravity Falls characters like Dipper and Mabel, and together they face the chaos of Weirdmageddon.
Definitely include Bill Cipher from Gravity Falls as he is the main villain in Weirdmageddon. Dipper and Mabel Pines are also essential as they are the main protagonists in the Gravity Falls story. If it's a crossover with the Over the Garden Wall universe, Wirt and Greg would be interesting additions. Wirt's cautious and somewhat gloomy nature could contrast with the more adventurous Gravity Falls characters, while Greg's innocence could add a different dynamic.
Maybe in the fanfiction, the three - tailed beast crosses over into a modern - day human world. People initially fear it, but then they discover that it has a gentle side and is actually trying to find its way back home. It could form a bond with a group of humans who help it on its journey, leading to many adventures along the way.
One idea could be a crossover with 'Fullmetal Alchemist'. Dorohedoro's dark and gritty world combined with the alchemical concepts would be fascinating. For example, the characters from Dorohedoro could be exposed to the laws of equivalent exchange. Another plot could involve a crossover with 'Hunter x Hunter'. The nen abilities in 'Hunter x Hunter' could interact in strange ways with the magic in Dorohedoro's world. Maybe a character from Dorohedoro has to participate in the Hunter Exam. A third idea is a crossover with 'Tokyo Ghoul'. The idea of the ghouls' existence in Dorohedoro's setting and how they would coexist or clash with the existing creatures and characters there.
One idea could be a Vegito crossover with the Dragon Ball GT universe. Vegito could face off against the Shadow Dragons, creating an epic battle of power. Another could be a crossover with the One Piece world, where Vegito has to navigate the Grand Line and deal with pirates. Maybe he teams up with the Straw Hats to take on a common enemy.
One idea could be a crossover with the 'Indiana Jones' series. The Mummy's curse could somehow be related to an ancient artifact that Indiana Jones is after. Maybe they have to team up to stop the mummy from unleashing a greater evil.
One idea could be a crossover with another popular space - themed series. For example, the Kyurangers could meet the crew of a well - known starship. They could team up to fight a common enemy that threatens both their universes, like an intergalactic pirate group. This would bring together different technologies and fighting styles.
One idea could be that the Turtles from 2003 get transported to the 2012 universe and they have to deal with the different versions of their enemies. For example, the Shredder in the 2012 version might have some new powers or tactics that the 2003 Turtles aren't used to.
One idea could be a crossover with a superhero show. Lee Ping could gain superpowers and have to learn how to use them while still relying on his wits from Detentionaire. Another could be a crossover with a time - travel show. Lee travels to different eras and has to solve historical mysteries that are somehow connected to his own world.
A really interesting plot could involve a new threat that emerges and can only be defeated by combining the powers and skills of the Persona 3 and 5 characters. For example, the combat styles of the two groups are different. The SEES members are more used to fighting in the Dark Hour, while the Phantom Thieves operate in the cognitive world. They would need to learn from each other's strategies. And on a character level, the more serious and reserved characters from Persona 3 could interact with the more flamboyant and confident characters from Persona 5, leading to some really interesting dialogue and relationship building.
A fun plot could be Arceus crossing over into a world of magic like in the Harry Potter universe. It could be a strange and interesting encounter as Arceus, a powerful Pokémon god, enters Hogwarts. The students and professors would be both amazed and a bit scared perhaps. And Arceus would have to navigate this world of spells and wizards while also trying to find a way back to its own realm.