Well, you could do a 'cartoon network war fanfiction' where the Teen Titans face off against the forces of the Red Guy from Cow and Chicken. The Red Guy could be trying to spread chaos across all the Cartoon Network universes, and the Teen Titans are the only ones powerful enough to stop him. They'd have to use their unique powers and teamwork to defeat his wacky and unpredictable attacks.
One idea could be a war between the Powerpuff Girls and the villains from Dexter's Laboratory. Maybe Mandark teams up with Mojo Jojo to create a super - weapon, and the Powerpuff Girls have to use all their strength and wits to stop them. Another idea is a war in the world of Adventure Time, where the Ice King tries to take over all the kingdoms in Ooo with an army of his ice minions. Finn and Jake would then lead a resistance against him.
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.
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.
One idea could be to have the cartoon versions of the heroes and villains swap powers. For example, Iron Man could end up with Thor's lightning powers and Loki could have access to Captain America's super - soldier strength. This would lead to really unique battles and character development as they struggle to control new abilities.
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 Time Squad could be on a mission to prevent a powerful magical artifact from Fairy Tail's past from being stolen in the present. They enlist the help of the Fairy Tail guild members who are experts in magic and know the history of the artifact. Together, they have to face off against a group of thieves who are also after the artifact for their own evil purposes.
Well, you could write a fanfiction about Naruto and his friends having to face an ancient evil that was sealed away long ago but has now been released. Or perhaps a story in which Naruto has to deal with the consequences of a jutsu gone wrong that affects the entire Hidden Leaf Village. You could also explore a fanfiction where Naruto teams up with some of the villains from the series and tries to understand their motives.
Well, a cool naruto cartoon crossover fanfiction idea might involve Naruto crossing over with the Avengers. Imagine Naruto's ninja skills and the Avengers' superpowers combined to fight a common enemy. Also, a crossover with the world of Fullmetal Alchemist could be interesting. Naruto could learn about alchemy and how it differs from his ninja techniques. And a Naruto and Attack on Titan crossover, where Naruto uses his skills to fight Titans.
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.
Another great Funny Cartoon Network Tumblr story could be related to the crossovers that fans dream of. Fans might post about how cool it would be if the characters from 'Ben 10' could team up with the ones from 'Steven Universe'. They would draw fan art or write short stories imagining the interactions between Ben's alien forms and Steven's gem powers, like Ben turning into an alien that can withstand the heat of a gem weapon and helping Steven in a battle.
Perhaps a story where the cartoon all - stars are invited to a grand party in a different dimension. But things go wrong when a mysterious figure starts causing chaos at the party. They then have to work together to figure out who it is and stop them. This could involve a lot of comical misunderstandings and cool team - up moments.