One idea could be that the characters from Cuphead end up in the world of Batim. Cuphead and Mugman might be mistaken for new ink creations in the studio. They could then have to deal with the strange and often menacing creatures in the Batim world while trying to find a way back to their own.
One idea could be a Cuphead crossover with Super Mario. Cuphead and Mugman could find themselves in the Mushroom Kingdom, facing off against Bowser's minions alongside Mario and Luigi. They could use their unique shooting skills to help in battles.
One idea could be that the characters from Mystreet end up in the world of BatIM. Maybe Aphmau and her friends are trying to find a way back home while being chased by the Ink Demon. Another could be a character from BatIM somehow enters the Mystreet universe and causes chaos among the residents there.
The art styles could be combined by having the characters keep their distinct looks but interact with environments that blend both styles. For example, Cuphead and Mugman could be running through a hall in the Batim studio that has the dark, inky look of Batim but with Cuphead - style geometric patterns on the walls.
One idea could be exploring Cuphead and Mugman's childhood. Maybe they had a special adventure before the events of the game. Another idea is to create a fanfiction where they meet new characters in a different part of the Inkwell Isles that has a unique theme, like an underwater world. A third idea could be a story where Cuphead has to face his fears without Mugman by his side for a change.
Another interesting concept is an OC that is a traveling musician in Inkwell Isles. This OC could meet Cuphead and Mugman during their travels and join them on their quests. They could use their music to soothe enemies or even as a form of attack, like creating sonic waves that stun bosses. It would add a unique element to the existing Cuphead world.
There could be a 'batim crossover fanfiction' that combines BATIM with a classic horror movie universe. Maybe the characters end up in the world of 'The Shining'. The dark and foreboding atmosphere of the Overlook Hotel in 'The Shining' could provide a great backdrop for the BATIM characters to navigate through, with new challenges and interactions that make for an exciting read.
One plot idea could be that Cuphead and Baroness have to team up against a greater evil in the Cuphead world. Maybe a new powerful devil - like character shows up and they realize they need each other's skills to defeat it.
One plot idea could be Djimmi and Beppi teaming up against Cuphead. They might create a series of wacky challenges in their own magical and carnival - themed worlds respectively, and Cuphead has to go through them to get a special item. Another idea is a story where Djimmi and Beppi start as rivals, each trying to outdo the other in terms of magic tricks or carnival attractions, but then they realize they can be more powerful together.
Perhaps Bendy and Cuphead are forced into a competition by a mysterious villain. They start off as rivals but gradually realize that they need to work together to defeat this common foe. This could involve them using their unique abilities, Bendy's flexibility and Cuphead's shooting skills, in creative ways.
Maybe in the fanfiction, there's a powerful villain who has stolen an important item from both Bendy's world and Cuphead's world. Bendy and Cuphead meet by chance and realize they have a common enemy. They start their journey together. Cuphead teaches Bendy some of his acrobatic moves, and Bendy shows Cuphead how to use the environment to his advantage with his ink.