Perhaps OK KO travels to Steven's universe and is amazed by the different types of gems and their abilities. He could learn from Steven about gem culture and in return, teach Steven some of his cool fighting moves. This could lead to them going on an adventure together to help a group of gems who are in trouble.
A possible plot could involve a gem experiment gone wrong that creates a threat that spans both universes. Steven, being an expert on gem-related things, and KO, who is always ready to save the day in his world, join forces. They meet at a neutral zone between their worlds, which is being affected by the chaos. Together, they have to find the source of the problem, which is a gem - machine hybrid that is sucking the energy from both universes. They encounter various challenges on their journey, like dealing with gem traps andobox security systems. Along the way, they also make new friends from both universes who help them in their quest to stop this hybrid threat.
A crossover with Adventure Time would also be cool. Ok Ko could end up in the Land of Ooo. He might team up with Finn and Jake to battle some sort of evil force that threatens both the plaza and Ooo. They could exchange their different fighting styles, like Ok Ko's formal karate training compared to Finn's more improvised sword - fighting.
Maybe a corrupted gem that has the ability to control technology. It sees the world of OK KO as a perfect place to spread its corruption. It takes overobox robots and uses them to attack both the heroes of OK KO and Steven. This gem villain could be really powerful as it combines the strength of gems and the technology of theobox world.
Enid from OK KO and Pearl from Steven Universe. Enid is a bit of a cool, laid - back character with her own brand of strength, and Pearl is a very refined and skilled gem. They could have a great dynamic, like Enid teaching Pearl some modern - day Earth things, and Pearl showing Enid some of her elegant combat moves.
One idea could be a crossover with another popular animated show like 'Steven Universe'. OK KO could end up in Beach City and have to team up with Steven to fight off a new type of gem - infected villains. They'd combine their unique powers, like OK KO's power fist and Steven's shield, to take on the bad guys.
One idea could be that Naruto and his friends end up in the world of Steven Universe. They could team up with the Crystal Gems to fight a new common enemy that has the power - stealing abilities like Orochimaru and the ability to corrupt gems like the Diamonds' powers. Naruto's jutsu could be a new and strange concept for the Gems, and Steven could try to understand Naruto's ninja way.
A really interesting plot could be that a powerful Gem weapon is stolen and ends up in the Voltron universe. The Paladins find it and don't understand its power at first. Meanwhile, the Crystal Gems think the Paladins are the ones who stole it on purpose. So there's this initial conflict. But as they start to communicate, they realize there's a bigger threat behind the theft. A shadowy figure is trying to use the weapon to destroy both universes. So they must join forces to stop this threat. The story could explore how the different characters interact and grow to trust each other.
One idea could be a crossover with "Adventure Time". Finn and Jake could visit Beach City and interact with the Crystal Gems. They could go on a mission together to stop a new threat that combines the magic of both universes.
One plot idea could be that Markiplier's character is a lost traveler who accidentally lands in the Steven Universe world. He is mistaken for a new type of gem at first. The Gems try to figure out what he is while he is just trying to find a way back home. But as time goes on, he starts to care about the Gems and their cause, and decides to stay and help them against the Diamonds.
One idea could be that Lars and Steven go on a space adventure together. They might encounter new gem species and have to work together to solve problems. For example, they could find a planet where the gems are in a civil war, and Lars and Steven use their unique skills to bring peace.