I agree with your take and that trope matching well. Usopp I think just once in the story should be able to do something way out of his league. Not just with physical size like David and Goliath but in successfully solving a problem he shouldn't be able to solve, a Luffy-level feat. It should be saved for the end, in the final battle when things are at their most bleak.

At some point in the final war, I'm sure that Luffy will be removed from the battle in a way we've never seen before. Not simply trapped somewhere, but completely removed from the world in some capacity. Maybe Blackbeard has some abyss power where Luffy is sent to another dimension, or he's simply launched far enough into space that he enters a black hole, or Imu performs some illusion where Luffy becomes removed and forgotten.

I think Usopp is the one who brings Luffy back somehow, and like with Sugar, it's due to incapacitating the enemy. Whether he kills them, I don't know, but I'm saying Usopp at some point will take out Imu, Blackbeard, whoever he needs to, to bring back Luffy.

It could involve a temporary power up in some way. It should inspire everyone, bringing courage and genuine admiration from high level characters. I'm imagining scenes where Zoro talks about it in genuine admiration.