Lapis Lazuli could be a great addition. She has a complex relationship with Peridot, and her presence could add some drama and depth. For example, she might be initially suspicious of Steven and Peridot's friendship, thinking that Steven is influencing Peridot too much. But over time, she could come to see the positive aspects of their bond.
Amethyst would also work well. Her carefree and fun - loving nature could contrast with Peridot's more logical and uptight personality. In the fanfiction, she could tease Peridot and Steven about their relationship, while also being there to support them when they face challenges. Maybe she helps them out during a battle against a new Homeworld threat.