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.
Perhaps there's a prophecy in both universes that foretells of a time when the Lions and the Gems must unite. However, there are forces in both worlds that oppose this union. The Voltron team and the Crystal Gems have to fight against these opposing forces while also trying to figure out how to combine their powers. Steven could play a key role in bringing the two groups together, using his ability to empathize and communicate. The story could involve a lot of character development as the two groups learn about each other's values and goals.
A crossover could start with Steven somehow getting his hands on a piece of Voltron tech. This could be something that fell through a wormhole. His curious nature would lead him to try and figure out what it is. Meanwhile, the Paladins of Voltron are on a mission to retrieve this tech. When they arrive in Steven's world, they are shocked to find a world full of Gems. The two groups could then learn from each other's powers and ways of fighting.
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.
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.
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.
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 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.
A more emotional plot could be an OC who was once part of the Homeworld gems but defected. They have a lot of guilt and self - doubt because of their past actions. In the fanfiction, they try to make amends by helping the Crystal Gems. But they constantly fear that their old allies will come after them. This internal struggle and their journey of redemption could be a really interesting story.
A cool plot could involve a crossover with a post - apocalyptic world. The Voltron team arrives and has to help the survivors there while also dealing with their own issues. They could share their technology to help rebuild the world, and in return, learn some survival skills from the locals.
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.
One possible plot could be that Steven and the Crystal Gems end up in Gravity Falls. They could team up with Dipper and Mabel to solve a mystery that involves both the magic of Gravity Falls and the Gem technology. Maybe there's a Gem hidden in Gravity Falls that has been causing strange phenomena, and together they have to find and deal with it.