One common theme is self - discovery. Many fanfics focus on Cheryl finding out more about herself, her strengths, and her weaknesses. For example, in 'Cheryl's Inner Journey', she embarks on a trip to understand her own emotions better. Another theme is family drama. Given Cheryl's complex family situation in the original series, fanfics like 'Cheryl and the Blossom Family Secrets' explore this aspect further. Love and relationships are also common, as seen in 'Cheryl's Romantic Escapades' where she experiences different types of love.
Well, a lot of these fanfictions deal with redemption. Maybe Jason has made some mistakes in the past and he tries to redeem himself in Cheryl's eyes. Also, mystery can be a theme. There could be secrets between them that they are trying to uncover. For example, a secret about their family heritage that only they can figure out together. And of course, friendship is also a theme as they are not only siblings but also friends in many of these stories.