Since we're posting theories...here's one:
Usopp will be the first to leave the Strawhats.
It'll happen on Sniper Island aka the homeland of his father Yasopp. It won't be on bad terms (more like bittersweet). It'll be a very sad moment.
Usopp will get a very high bounty after Dressrosa, potentially higher than Luffy's. His name will become notorious across the New World and he'll become a famous pirate, even more-so than his father.
He creates a massive crew of various gladiators and dwarves from Dressrosa, samurai from Wano, giants from Elbaf, and snipers from Sniper Island and eventually comes back towards the end to help the the Strawhats in the Great Pirate War.
Not only that but he'll become a Yonkou before Luffy after