Maybe the characters are brought together in a strange new world that's a mix of their own. The Powerpuff Girls could meet the Teenage Mutant Ninja Turtles. The Girls' superpowers and the Turtles' ninja skills would make for an exciting dynamic. They could fight against a new villain that threatens this combined world.
I think Squidward from Nicktoons and Courage the Cowardly Dog from Cartoon Network would be quite unexpected. Squidward is always so grumpy and pessimistic, while Courage is, well, cowardly but also very kind - hearted. Their interactions would be really strange and comical.
One idea could be to have the heroes from Cartoon Network shows like Ben 10 and the Powerpuff Girls team up to fight against the forces similar to those in Infinity War. Ben could use his various alien forms to take on different challenges, while the Powerpuff Girls use their superpowers. They could also meet some of the Marvel characters and have some comical yet action - packed interactions.
One interesting concept could be the meeting of different Cartoon Network characters from various shows. For example, the characters from Adventure Time and Regular Show coming together to solve a problem in a shared universe. This allows for unique interactions and the exploration of how different personalities would mesh.
Danny Phantom is also highly favored. His ghost - fighting abilities and the fact that he has a double life as a normal teen and a superhero add a lot of depth. He could use his powers to help or protect other characters in the crossover.
One idea could be that all the Nicktoons characters are brought together to fight a new, powerful villain that threatens all of their universes. They have to combine their unique powers and skills to stand a chance.
First, choose the characters from different Cartoon Network shows that you want to include. For instance, if you like the humor of Chowder and the adventure aspect of Samurai Jack, pick them. Then, think of a simple plot like they get lost in a strange dimension and have to find their way back home. This gives a basic structure to start with.
Some cool Cartoon Network fanfiction might involve Ben 10 in a situation where he loses his Omnitrix but still has to fight aliens using his wits and the knowledge he's gained from using it. There could be a fanfic about Dexter's Laboratory where Dexter teams up with Mandark to fight a common enemy that threatens their entire city. Also, a fanfiction about Courage the Cowardly Dog where he has to save Eustace and Muriel from a time - traveling menace would be really exciting.
The theme of unity against a common threat is also seen. When there's a problem in the city, like a villain or a natural disaster, all the characters from different shows have to work together. It shows how diverse characters can come together and use their unique abilities to save Cartoon Network City. This theme not only makes for exciting stories but also promotes a sense of togetherness.
First, choose your favorite Cartoon Network show. Let's say Adventure Time. Then, think about the characters you love, like Finn and Jake. Decide on a simple plot, for example, Finn loses his sword and has to go on a quest to find a new one. You can start by writing a short introduction about Finn's situation. Just write down your ideas as they come to you, don't worry too much about grammar at first.