A common theme is friendship - turned - love. They start as friends and gradually develop deeper romantic feelings. Another theme is mutual support. They help each other through tough times, like Haru being there for Ryuji when he has family issues or Ryuji giving Haru confidence in social situations.
One common theme could be redemption. Given Kamoshida's actions in the game, fanfic might explore his path to redemption in relation to Ryuji. Another theme could be friendship or an unlikely bond forming between them.