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.
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.
Rebecca could potentially be in Luffy's harem. After Luffy helped her in Dressrosa, she might develop feelings for him. She is a strong - willed and kind - hearted character. Another could be Shirahoshi. Luffy has a special connection with her as he saved her from being captured. Her gentle and innocent nature would contrast with some of the other more assertive members of the harem.
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.