Friendship turned to more is a very common theme. Since they start as crewmates with a normal friendship in the original, fanfictions often explore how that can develop into a romantic or deeper emotional bond.
One great Zoro x Usopp fanfiction is 'The Unlikely Bond'. It beautifully explores their friendship evolving into something more. The author does a wonderful job of depicting Zoro's tough exterior softening towards Usopp, and Usopp finding new courage in Zoro's presence.
In some 'usopp fanfiction', he might be the key to solving a mystery on the ship. Maybe there are strange occurrences that only his unique perspective and skills can figure out. This could involve his talent for creating gadgets and his sharp observational skills, which are often overlooked by others but prove crucial in these fictional scenarios.
One common theme is empowerment. Since Usopp is often seen as a bit of an underdog, in female Usopp fanfiction, she is often empowered to be a strong and capable character. Another theme is friendship. Just like in the original story, the female Usopp forms strong bonds with her crewmates. Self - discovery is also a theme. She may discover her own abilities and what she truly wants in life through her adventures.
One popular Usopp fanfic might be those that focus on his growth as an inventor. For example, stories where he creates new and amazing gadgets for the Straw Hat crew. Another could be fanfics that explore his relationship with other crew members in more depth, like his friendship with Luffy and how they support each other during tough times.
I recommend One Piece: The Great God Usopp to you. This book is a story about Usopp. It tells the story of how he obtained the League of Legends system through an accidental transmigration and became invincible. If you want to read a One Piece Doujinshi novel with a deceptive cloth as the main character, this book is perfect for you. I hope you will like my recommendation.