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.
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.
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.
Some dedicated Steven Universe fan communities might have fanfiction archives or threads dedicated to this pairing with the hurt/comfort genre. For example, certain Discord servers focused on Steven Universe fandom. Members often share and discuss their favorite fan works, and it's possible to find some great Lapis Lazuli x Peridot hurt/comfort fanfics there. You just need to join these communities and start exploring.
A good one often has a believable scenario where either Lapis or Peridot is hurt, either physically or emotionally. For example, Lapis might be dealing with past traumas from her time trapped in the mirror, and Peridot is there to comfort her. The characterizations should be on - point, so they act like they do in the show.
Finding "peridot x lapis lemon fanfic" can be a bit tricky. Some fanfic communities on social media platforms might be a good place to start. You can ask fellow fans if they know of any good ones. Also, make sure to respect the rules and guidelines of each platform when looking for fanfic, especially if it involves certain pairings.
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.
Some Steven Universe - dedicated fan forums might have fanfictions as well. Check out places like the Steven Universe Reddit community. Sometimes fans share their own works or recommend good ones there.
One great fanfiction could be 'The Bond of Lapis and Peridot'. It explores their relationship in a new adventure on Earth. They face challenges together and their unique personalities play off each other really well in this story.