One possible fanfiction could be about Lapis and Peridot fusing during a time of great danger on Earth. They fuse to protect their friends and the planet from a new, powerful enemy. Their combined powers create a unique and beautiful form that has abilities neither of them had alone.
The plot could involve Lapis and Peridot fusing accidentally at first. Maybe they are trying to work together on a project, like building a new structure for the Crystal Gems' base. Their powers interact in an unexpected way and they fuse. Then, they have to figure out how to function as this new being while also dealing with the reactions of the other Gems.
Since the concept of 'abuses' is not family - friendly in itself, it's very unlikely to find such fanfictions that are also family - friendly. Fanfiction should be a medium to tell positive stories, especially when it comes to characters that are often loved by a family - friendly audience like those from Steven Universe. So, I would say no.
I'm not sure there should be fanfictions about 'abuses'. Fanfiction should generally promote positive and healthy relationships. However, if you mean interactions between Lapis and Peridot in a non - abusive way, you can look on popular fanfiction platforms like Archive of Our Own. Search for stories tagged with their names and explore the ones with high ratings and positive reviews.
One interesting story could be about Peridot and Lapis exploring a new planet together. They might encounter strange creatures and have to use their unique powers to survive. For example, Peridot's technical skills could help them build a shelter while Lapis uses her water powers to find a water source.
One common theme is friendship. Since they are both characters, their relationship as friends is often explored. Another theme could be redemption, perhaps one of them has made a mistake and the other helps them through it.
Another interesting one is 'The Tale of Lapis and Peridot'. In this story, they get involved in a mystery on Earth. The author does a great job of portraying their personalities and the way they interact with the human world.
There's 'The Adventures of Peridot and Lapis' which is quite popular. It follows their journey across different dimensions, facing various challenges. It has great character development for both of them.
One common theme might be the exploration of their relationship dynamics. Since they are two distinct characters, fanfic might focus on how they interact and get to know each other better. Another theme could be adventure, where they go on some sort of journey together.