I thinks this is plot bunny but please correct me if im wrong.
The "lost" prince and his noble marksman au. Ftm Sanji. Down bad Usopp.
Usopp was from Germa as well. Yasopp was knight how fled to sea after he was tried for treason, for being seen with Shanks.
Judge took Usopp from his mom as her punishment. Usopp was groomed to take over his father's old job of royal marksmen.
When Sanji gets thrown in the dungeon, Usopp and Reiju free him. Usopp stays with Sanji of his own free will. He never leaves Sanji because Sanji would never leave him.
When Sanji realizes he has feelings Usopp he freaks out because of the power imbalance he sees. He pulls away from Usopp. Everyone on the ship sees it but Usopp feels it.
Usopp stops acting like himself and shuts down after the first week. He knows how Sanji gets sometimes, but this time feels different. Usopp is deeply afraid Sanji found out he likes him more then a common born marksman has a right to love his prince.
Nami has to step in when Usopp stops coming to meals. She snaps at Sanji. Explaining she doesnt know what Usopp did but clearly he's learned his lesson. She tells Sanji to go talk to Usopp and fix whats going on. Thats when she learns the truth
Sanji brakes down about everything. His childhood, what his father did, how Usopp was so loyal to his prince he left his home and almost died a million times along the way. He screams about how wrong his feelings are because he knows at the end of the day Usopp still sees Sanji as his prince. That him acting on his feelings should be wrong because of their power imbalance.
Luffy is having a similar conversation with Usopp. The truth of how they know each, how Sanji was and is his prince. He explains how no prince wants the son of a traitor and commoner. That if Sanji ever got with him, it would be out of pitty or worse an unfounded "debt" to Usopp. He wants his prince's love. He sees this as his prince has decided to punish him for overstepping.
It comes to a boiling point when Usopp gets hurt in a fight, because his malnutrition from not eating the food Sanji cooks. Sanji hears Robin yell Usopp's been hurt and calling for Chopper. The full force of his training comes out. The flamed footed prince is on a rampage. The Strawhats are all a little scared, even Zoro gives him some room. He single handedly eradicate everyone they crew was fighting. When get gets his hands on the person who hurt Usopp its the most sadistic thing anyone's seen him do. It was almost like he was playing with their life.
Chopper gets Usopp stable and Sanji never leaves his side. He speaks to Usopp in the native language of Germa. Praying he stays with him and making promises. Usopp had been out so long Sanji slips up and confess his feeling for him. Begging Usopp to get better. Apologizing for pulling away. Saying he just never wanted to make Usopp feel he had to allow Sanji to do anything he wanted because of his title. He knows Usopp still sees him a prince, due to him being called "the prettiest royal cook" by Usopp.
Usopp wakes up to Sanji's confession and freezes. He's died and gone to heaven. His beloved Sanji is saying such thinks. He blushes and Sanji plays with his hair redoing his locks as he lays there. Sanji pulls a bit to hard and Usopp moans. He losses himself and flips to kiss Sanji. To hell with it, this is his peice of heaven. He gets to explore every single inch of Sanji finally.
Sanji kisses back instantly. Soaking in the man he's loved for so long taking control like he was made to it. His hand scramble for purchase and he fumbles to hold Usopp.
Usopp hums into Sanji's mouth as he so sweetly trys to finds something to grab. His sweet Sanji. Usopp guides Sanji's hands to his hair as he places a hand on Sanji's neck to pull him closer.
Sanji's about to go for the straps of Usopp's overalls, when Nami walks in. Usopp becomes very aware, very quickly this is real life, he's not dead, and Sanji really actually wants him. Usopp picks up a near by pillow and throws it at Nami causing her to laugh as she leaves with a reminder of being safe.
Usopp grabs Sanji's waist to bring his back from when he backed away from him after Nami scared him. Speaking back in their native tongue. Saying " Oh, no you don't pretty boy. I wake up to your fingers in my hair while you tell me such pretty little promises. Now I'm gonna finish what I started. I've always dreamed of such things since Zeff first took us in. I'm not letting you go anywhere."
Sanji melts into the pull. He smiles sweetly as Usopp plays with his bangs as he says such things with such sincerity. He apologizes for letting things get so far out of fear. Well he tries to but he has a very skilled mouth going down his throat, making thinking and talking hard. He squirms as he feels Usopp smile against his collar bone.