Making things out of human junk
Ussop representing Sanji's dream
Love the variety of thoughts here 😭
I always perceive Sanji as the mermaid, but Usopp's story has so much potential here too... (EDIT after writing the whole thing: This wasn't meant to be a Little Mermaid AU and just a mermaid AU in general but the whole thing fits so well, I had to mention it...) - So Little Mermaid AU it is !
Usopp coming from a sea region that fears humans and whatever they might do to them if they're ever found... His mom being sick and both her and Usopp being the only ones knowing Yasopp is a human pirate... Usopp always claiming the humans are coming for them... Yadda yadda yadda, much to think about here, but you get it.
And so Usopp longs for the human world too, curious of what might bring since he's always making things out of human junk (as you said, because that is something he'd 100% do). He tells everyone he's been to the surface and knows first-hand stories from there. He loves to tell lies people will believe (his best friend Kaya will, at least) about things he knows nothing about but wishes he would.
AND!!! You know the drill, Little Mermaid stuff!!!
I think Sanji almost drowning with Zeff exactly like what happened in canon except that he can't fully save him and little mermaid Usopp helps them out and saves Sanji... And it becomes a whole legend that Zeff doesn't quite believes even if he saw it with his own eyes... And Sanji wishes to find the All Blue because he believes that if he does, he will ALSO find the mermaid that saved him... God... Adding childhood sweethearts to this I AM CRAZY ABOUT THEM!!!!!! Then they fall in love and y'know, Little Mermaid plot kinda...
(I just thought the whole "not being able to speak" thing in the Little Mermaid fits them so well here because Usopp won't shut up and his best skill is talking and HE CAN'T so... Yeah...).
The point is that Usopp is not only the representation of Sanji's dream in a tangible, physical way (the sea, finding the All Blue, mermaids, stories...) but also in the sense of freedom. Sanji finds mermaids mesmerizing because they can swim anywhere. For Sanji, the sea is an escape from his past life and he wishes he could be as peaceful as he thinks he'd be underwater (a place he KNOWS by heart and wishes to know MORE). So Usopp is not only Sanji's dream of finding a mermaid or the All Blue, but also the representation of what it would be like to be free.
While Sanji represents Usopp's dream of the unknown. Of all the stories he makes up and wishes could become real someday. Of standing up for himself and proving to everyone else that he can build magnificent stuff the way humans have, and prove that not all of them are evil. To make his father proud, also. And Sanji, with his relationship with Zeff and skills, is everything Usopp wishes to be.
(Kinda wondering how Usopp feels about Sanji cooking fish but you know what I'm not thinking about it!)
And YES shirtless mermaid Usopp. What a very nice, awesome thought that would send Sanji into a cardiac arrest.
OH!!! And the concept of Ursula not only being Big Mom but also adding Pudding to the whole thing for her to manipulate Sanji and pretend she was the one to save him, etc etc... Then she feels awful and has a redemption arc but that's because I love her!
Special mention to how "Part Of Your World" is soooo Usopp coded:
The dreaming, the story-telling, the ambition, the envy... I need to be put down-
While "Wild Uncharted Waters" is literally Sanji's song to Usopp: