Blue Diamond is another popular one. Her emotional nature in contrast to her position of power on Homeworld offers a lot of room for exploration in fanfiction. Writers can delve into her grief over Pink Diamond's loss and how that affects her decisions regarding Homeworld and its future, as well as her relationships with other Gems both on and off Homeworld.
Yellow Diamond is also quite popular. She represents the power and authority of Homeworld. Fanfictions might focus on her character development, for example, showing a softer side of her or exploring how she came to be so strict in her rule. Her interactions with other Diamonds and her role in the overall hierarchy of Homeworld are interesting aspects to write about.
The Diamonds are extremely popular in gem war fanfiction. Their power and their role in starting and perpetuating the war are fascinating to explore. Writers can delve into their motives, their relationships with each other, and how they react to the events of the war, whether it's through changing their views or doubling down on their actions.
Obviously, Goku and Steven Universe themselves are very popular. They are the main characters that people are excited to see interact. Their different personalities, Goku's bold and straightforward nature and Steven's empathetic and kind - heartedness, make for great chemistry.
There could be a fanfiction where a Gem from Homeworld discovers the beauty and freedom on Earth and decides to rebel against the traditional ways of Homeworld. This could involve making friends with the Crystal Gems and learning about love and acceptance in a new way. For example, they might start to question the purpose of the Gem hierarchy and fight for equality among all Gems.
Steven is definitely one of the most popular. Since he is the main character in the show, many fan fictions revolve around his further adventures and growth. Pearl is also quite popular. Her elegance, intelligence, and complex relationship with Rose Quartz give writers a lot to work with. Garnet is another favorite. Her stoic nature and the mystery behind her fusion are often explored in fan fictions.
Peridot is quite popular too. Her unique personality, especially her growth from a somewhat antagonist to a more friendly character, makes for great storytelling in crossovers. In a crossover with 'Ben 10', Peridot's technological knowledge could be paired with Ben's Omnitrix - related adventures.
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.
Steven himself is of course very important. He is usually the central character trying to break the time loop or understand its purpose. His unique powers and kind heart are often key elements in the story.
A popular pairing could be Dipper and Steven. They are both curious and adventurous, so fans might like to imagine them exploring new worlds together. Their different backgrounds would make for an interesting dynamic.
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.
One popular trope is the fusion of Pokemon with the Gems from Steven Universe. For example, a Pikachu - like Gem or a Gem - powered Pokemon. It combines the unique powers and characteristics of both universes in an interesting way.