A more comical plot could be that Cuphead accidentally ends up in Baroness' lair and she mistakes him for someone else. Cuphead then has to play along to avoid getting into trouble, and during this time, they have all sorts of wacky adventures together. This would be a fun, light - hearted take on the Cuphead x Baroness relationship.
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.
Well, perhaps there could be a plot where a mysterious villain is causing chaos in both of their worlds. Cuphead and Bendy meet and initially don't trust each other, but as they learn more about the villain's plan, they realize they need to work together. They could start to teach each other about their own unique skills and cultures along the way, like Cuphead showing Bendy how to use his shooting skills better and Bendy teaching Cuphead some sneaky tricks he learned in his spooky world.
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.
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.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions on various topics, including Cuphead x Baroness. Just use the search function on the site to look for them.
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.
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.
One plot idea could be King Dice getting into a tricky situation where he has to choose between loyalty to the Devil and his own sense of right and wrong. Maybe he discovers a plan of the Devil's that he doesn't agree with, like a scheme to cheat more souls out of the Inkwell Isles' inhabitants in a really unfair way. Another idea is a backstory for King Dice that shows how he became so close to the Devil in the first place. It could involve his own ambitions and how the Devil promised to fulfill them. A third plot could be about a character from the Cuphead world trying to save King Dice from the Devil's influence, leading to a series of challenges and adventures.
Friendship is another common theme. They could start as enemies or strangers and then develop a friendship. Fanfictions might show how they learn to trust each other, help each other out in difficult situations, and how their friendship changes them both for the better.
One idea could be a story where Snake Eyes is on a mission to retrieve a powerful artifact from Cuphead. He has to face all of Cuphead's tricky challenges and minions. Another plot could involve Snake Eyes teaming up with Cuphead to take on a common enemy that threatens their world. Maybe a powerful demon that wants to take over their land.