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.
One idea could be a story where Sanji and Usopp get stranded on a strange island together. Sanji has to rely on Usopp's creativity to find food sources that are not immediately obvious, while Usopp depends on Sanji's combat skills when they encounter dangerous creatures on the island. This would force them to bond and understand each other better.
Sanji's chivalrous nature can be further explored. For example, when he and Usopp are in a dangerous situation, Sanji could put himself in harm's way to protect Usopp, which shows his loyalty and sense of honor. Usopp, on the other hand, can develop his courage. Maybe he stands up to a powerful enemy to protect Sanji, overcoming his usual cowardice and growing as a character.
One idea could be that Sanji and Usopp team up for a cooking competition on the ship. Sanji brings his world - class cooking skills and Usopp uses his creativity to come up with unique presentation ideas. Another could be a situation where they get lost on an island together and have to rely on each other's skills, Sanji for survival cooking and Usopp for building shelters and creating distractions against potential threats.
I would recommend 'Three Souls, One Dream'. It has an interesting take on their interactions, showing how Zoro, Luffy, and Sanji grow together as a team. It has some really heartwarming moments that capture the essence of their friendship.
I would recommend 'Ace, Luffy and Sabo: Reunion of Souls'. This fanfiction is full of heart - warming moments. It shows how their past experiences shape their present interactions and how they support each other in the face of difficulties.
Their relationship could develop through mutual respect. Luffy respects Whitebeard as a powerful and respected pirate, and Whitebeard respects Luffy's determination and potential. For example, they could start with a simple exchange of stories about their adventures at sea.