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.
Here's another thought. The Loud sisters could go on a beach vacation where they accidentally cross into the world of Spongebob. They'd be amazed by the underwater world. Lori might try to use her phone underwater, which of course wouldn't work. And the sisters could team up with Spongebob and his friends to stop Plankton from stealing the Krabby Patty formula this time.
Well, a cool plot could be that Lincoln Loud visits Bikini Bottom. He might get involved in Spongebob's adventures at the Krusty Krab. He could help Spongebob deal with Mr. Krabs' money - loving ways. And perhaps he'd introduce the idea of a big family to Spongebob and his friends, which is so different from what they know in the ocean.
A cool plot could be that Mr. Krabs discovers a new market in Royal Woods. So he sends Spongebob and Squidward there to set up a Krusty Krab branch. The Loud kids are excited about the new restaurant, but Squidward is miserable as always. Lincoln and Spongebob team up to make the new Krusty Krab a success, facing challenges like dealing with local competition and getting used to the different customers in Royal Woods.
Well, maybe there's a portal that opens up between Royal Woods and Bikini Bottom. Spongebob and Patrick come through to the Loud House world. Patrick could be really confused by all the human stuff and keep getting into funny situations like trying to eat the furniture thinking it's food. Meanwhile, Spongebob and Lincoln team up to solve a mystery that involves a missing item from both their worlds. It could be something like a special kind of jellyfish that has magical powers and it has ended up in Royal Woods.
A cool plot for 'the loud house crossover fanfiction' might involve a crossover with 'My Little Pony'. The ponies could come to Royal Woods, and the Loud sisters could be fascinated by their magic and cuteness. Lincoln might have to figure out how to get the ponies back to Equestria while also dealing with his sisters' excitement. Or perhaps the Loud family travels to Equestria and has to adapt to a world ruled by ponies.
One idea could be that the misfortunes from 'No Such Luck' somehow cross over into an alternate universe in the crossover. For example, Lincoln's bad luck could start affecting the other characters from the crossover world in strange ways, like causing their gadgets to malfunction.
One idea could be a crossover with another popular animated show. For example, the characters from 'Loud House' could end up in a different universe where they have to use their unique skills from 'No Such Luck' to survive. Maybe they meet characters who have similar family dynamics but different challenges.
One idea could be that the Loud siblings discover they have magical abilities and are sent to Hogwarts. Lincoln could be sorted into Gryffindor for his courage in dealing with his large family. Another could be that a wizard from the Harry Potter world visits Royal Woods and gets involved in the chaos of the Loud family. Maybe they help the family solve a mystery using magic.
One idea could be that Lincoln Loud discovers a rare and powerful Pokemon in Royal Woods. He then has to keep it a secret from his sisters while training it. Another could be the Loud sisters each getting a Pokemon that reflects their personalities. For example, Lola might get a Jigglypuff because it's cute and girly.
One idea could be that Naruto somehow ends up in the Loud House world. Maybe he's on a mission and a strange jutsu goes wrong, transporting him there. He could use his ninja skills to help the Loud siblings out with their daily problems, like getting rid of a bully at school using his shadow clone jutsu.
One idea could be that the Loud family members gain superpowers similar to the Avengers and have to team up to save Royal Woods from an alien invasion. For example, Lincoln could be like Captain America, leading the group with his strategic mind.
I think Lincoln and Spongebob would be an interesting pairing. Lincoln is used to living in a big, chaotic family and Spongebob is always full of energy and optimism. They could get into all sorts of misadventures together, like exploring a new part of Bikini Bottom or trying to solve a mystery in Royal Woods.