Au where ace becomes captain after luffy dies (straw hat ace au)
Ace likes usopp a lot because he reminds him of a young luffy (wants to gets stronger, a bit scared but when you really need him he stands strong) also like luffy, ace sees usopp as a silly little guy.
So I like to imagine ace starts acting like a protective older brother to usopp (unlike luffy, he senses usopp needs to be protected)
This is actually a point of contention between him and the crew! Ace babys them. This takes a while to fix especially considering the whole crew is a little silly and childish around eachother because that's how they were when Luffy was there. Ace keeps stealing their fights every time they get into one just because Ace can cut through some of the earlier enemies like butter because he's been a New World Pirate for years. Everyone is annoyed by this because they can't have their Moment of looking awesome because the second even a marine choreboy pulls a sword on one of them Ace is already there overdoing things because he's got the social awareness of a matchbox and the reasoning of a stick of dynamite
