In Cheryl Blossom OC fanfiction, a frequently seen theme is the power dynamics. Cheryl comes from a wealthy and influential family in Riverdale, and when an OC enters her world, it creates an interesting shift in the power balance. Sometimes the OC is from a different social class, and how they navigate this difference with Cheryl is a big part of the story. Another common theme is family. The OC might have a different family situation compared to Cheryl, and how their families interact or how the OC affects Cheryl's relationship with her own family can be a central part of the fic. There's also the theme of mystery. Some fics make the OC have a mysterious past or purpose in Riverdale, and Cheryl gets drawn into solving that mystery.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including Riverdale where Cheryl Blossom is from. Just search for 'Cheryl Blossom' in the search bar.
One great Cheryl Blossom fanfiction could be one where she overcomes her family's dark secrets and finds true friendship. It might focus on her growth from a somewhat villainous character to a more complex and sympathetic one.
Jason Blossom is another possibility. Even though he has passed away in the show, in fanfiction, there could be a supernatural or alternate - reality plot where he is the father. It would add a lot of drama and mystery.
Another common theme is friendship. Before they become romantically involved, they often start as friends. Fanfictions explore how their friendship blossoms into something more. For example, they might support each other through family issues or problems at school in the stories.