Love and romance are very common themes. Usually, the story will revolve around the relationship between the rrb character(s) and the reader, with a focus on how they fall in love.
Adventure can also be a theme. For example, the rrb character and the reader might go on a journey together, facing challenges and growing closer through shared experiences. Another common theme could be self - discovery. As the reader interacts with the rrb character, they might learn more about themselves, their strengths, and their weaknesses. It's often a way for the author to explore the character development of both the rrb character and the reader in the context of their relationship.
Some rrb x reader fanfiction might have a theme of mystery. Maybe the rrb character has a secret that the reader is trying to uncover. Or there could be a mystery in the world they live in that they both have to solve. This adds an element of intrigue to the story and keeps the reader engaged as they try to figure out what's going on along with the characters.