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.
I'm not entirely sure as there could be various interpretations. It might be a novel centered around Zoro from One Piece. It could focus on his backstory, his training, or his adventures separate from the main One Piece storyline.
One great fanfiction could be about their adventures on a new island not shown in the original story. Maybe they encounter a powerful and strange new enemy. Luffy's determination, Zoro's strength, and Sanji's culinary skills all come into play in this new situation.
One possible storyline could be that after a long separation, Usopp returns to Syrup Village as a hero. Kaya, overjoyed to see him, runs into his arms and they share a spontaneous kiss. This kiss could be the start of a new chapter in their relationship, with Usopp finally realizing his feelings for Kaya are more than just friendship.
Well, in such a fanfiction, there could be various hints. Maybe Zoro's behavior might be more in line with typical female characteristics described in that specific fictional world, like being more empathetic or having different social interactions compared to the original male - character version. But it really depends on how the author has crafted the story.