In a crossover with "Avatar: The Last Airbender", Aang would be a great addition. His ability to bend all four elements could interact in interesting ways with the Gems' powers. For example, his airbending could work with Pearl's grace and agility, or his waterbending could have an effect on the water - based Gems.
For a crossover with "The Owl House", Luz would be an excellent character to include. Her knowledge of glyphs and her eagerness to learn magic could lead to her learning about Gem magic. She could also introduce the Gems to the magic of the Boiling Isles, and together they could face a threat that spans both their worlds.
Another good opponent could be a character like Naruto from Naruto. Their powers are very different. Naruto has his ninjutsu and chakra, while Steven has his gem abilities. The clash of their ideals and fighting styles would make for a great fanfiction Death Battle.
Lars himself is of course a must - he's the central character in 'Lars of the Stars'. Steven is also very popular as he has a strong connection to Lars in the original series. His positive attitude and unique powers can add a lot to the fanfiction. Sadie could also be included as she has a special relationship with Lars. Her character development and her own skills could be explored further in the fanfiction context.
One interesting crossover could be Naruto characters in the world of Steven Universe. For example, Naruto's ability to use chakra could be a unique power in the SU universe. Maybe he could use his Shadow Clone Jutsu to help out in a gem battle. Another could be Steven trying to understand Naruto's ninja way and learning some of his moves to defend Beach City in a new way.
Another possibility is a crossover with "Gravity Falls". Dipper and Mabel could come to Beach City during a mystery involving Gem technology. Their knowledge of the strange and unusual from Gravity Falls could combine with Steven's understanding of Gem lore to solve the mystery.
Pearl would be an interesting character to include. Her grace and combat skills, combined with her complex backstory, could add depth to any crossover. In a crossover with a historical - themed show, Pearl's knowledge of different eras could create interesting storylines. Another interesting character is Garnet. Her fusion status and stoic nature can lead to unique interactions, especially in a crossover with a show that also has characters with dual identities or special powers.
He would probably have a very mixed reaction. On one hand, seeing how fans view his world and his story could be really cool. He might enjoy seeing different versions of himself and his friends. However, if there are inaccuracies or things that go against his values in the fanfiction, he could also be a bit put off. It really depends on the nature of the fanfiction he's reading.
One interesting crossover could be the use of magic in the Steven Universe world. For example, Steven might learn some basic spells from the Harry Potter universe and use them in his own world to solve problems in a new way. Maybe he could use a levitation spell to move the big rocks around on the beach more easily.
Well, in a Steven Universe and Spider - Man fanfiction, there could be a situation where the Infinity Stones from the Marvel Universe are somehow related to the Gems in Steven Universe. Spider - Man gets caught up in this mystery. He teams up with Steven and the other Gems to figure out how these powerful items are connected. Along the way, they face challenges like villains who want to use the combined power of the Stones and Gems for evil. This kind of crossover would bring together two very different but equally fascinating universes.
One popular crossover is having the Crystal Gems from Steven Universe team up with the Avengers to fight a common enemy. For example, they could battle a powerful alien threat that neither group could handle alone. The unique powers of the Crystal Gems, like Garnet's future vision and Steven's healing powers, combined with the Avengers' superhuman abilities would make for an exciting story.
In some fanfics, there could be a situation where Steven visits the Naruto world. He might be really confused at first by all the ninja stuff. But then he could start to use his Gem powers in creative ways to help out. He could heal injured ninjas with his healing spit, and the ninjas would be amazed at this strange power from another world.