How about a crossover with the Powerpuff Girls? The Powerpuff Girls could come to Bikini Bottom to help fight Plankton when he creates an especially evil plan. Bubbles might befriend Spongebob because of their innocent natures, while Buttercup and Patrick could have a funny rivalry.
One idea could be a crossover with the Teenage Mutant Ninja Turtles. Picture Spongebob and his friends in the sewers of New York, trying to help the turtles fight Shredder. Maybe Squidward could use his clarinet as a weapon in a comical way.
A Spongebob and Harry Potter crossover would be really cool. Spongebob could visit Hogwarts and cause chaos in the hallways. He might accidentally turn himself into a fish - human - sponge hybrid with a misfired spell, and Patrick could be the bumbling sidekick who gets into all sorts of trouble with the magic.
One idea could be that Spongebob and Patrick end up in the Loud House world. They could be really confused by all the human stuff and the big family. Maybe they try to make a Krabby Patty in the Loud House kitchen but use the wrong ingredients and chaos ensues.
One popular idea is a crossover with other Nickelodeon shows like "Avatar: The Last Airbender". Imagine Spongebob in the world of bending elements! Another is a crossover with "Teenage Mutant Ninja Turtles". They could have underwater adventures together in Bikini Bottom. Also, a crossover with "Fairly OddParents" where Cosmo and Wanda visit Bikini Bottom and cause some magical chaos.
One idea could be a story where Spongebob and Patrick go on an adventure to a new underwater kingdom that no one in Bikini Bottom has ever visited. They could encounter strange new creatures and have to use their friendship to get out of tricky situations.
Some great 'Spongebob crossover fanfic' ideas include a crossover with 'Scooby - Doo'. The gang could visit Bikini Bottom to solve a mystery that involves Spongebob's pineapple house. A crossover with 'Digimon' is also possible. Spongebob and the gang could enter the digital world and befriend some Digimon. And a crossover with 'Powerpuff Girls' would be cool. The Powerpuff Girls could fly to Bikini Bottom to help Spongebob fight some new underwater villain.
One idea could be that Squidward gets turned into a vampire. He then tries to turn Bikini Bottom into a spooky vampire realm, but Spongebob, being his usual positive self, tries to stop him and turn him back to normal with the power of friendship and some kelp - made anti - vampire potions.
One idea could be that Spongebob and Squidward accidentally switch personalities. So Spongebob becomes grumpy and Squidward becomes overly enthusiastic. It would lead to a lot of comical situations in Bikini Bottom.
One interesting idea could be Spongebob as a Jedi. Imagine him using his spatula as a lightsaber. Another could be Mr. Krabs being a greedy Sith, always after more power in the Star Wars universe. And Patrick might be a clumsy but well - meaning sidekick like R2 - D2, getting into all sorts of comical situations while trying to help Spongebob on his adventures.
Well, perhaps in the fanfiction, Spongebob and Pearl could start a business together. Since Pearl loves lemons, they could open a lemonade stand in Bikini Bottom. But they have to deal with Mr. Krabs who is jealous of their success. Or, they could go on an adventure to find the rarest lemon in the ocean, which is said to grant wishes. Along the way, they meet new friends and enemies.
One interesting idea could be how Squidward's character might change during the pregnancy. He's usually grumpy and reserved, but pregnancy could bring out a more vulnerable and emotional side of him.
Another idea is a story where Sandy wants to introduce some of her land - based inventions to the underwater world. Spongebob helps her with the process, but things go hilariously wrong at first. For example, a machine that is supposed to make the water around Bikini Bottom cleaner ends up making it all muddy. They then have to work together to fix it and learn a valuable lesson about testing things properly before using them in a new environment.