I feel like this little bit in the Luffy/Sabo/Ace flashback is routinely forgotten and remains really overlooked.
I was rereading the flashback recently and stumbled over it again surprised.
If the images don't show up for you, the chapter is 585 (volume 60). The 12th and 13th pages.
The boys are yelling out their ambitions in front of eachother. Ace goes first, then Luffy takes his turn and…
...Oda cuts away ...and cuts back to Ace and Sabo's reaction of confusion and surprise.
What the heck could Luffy have said that would cause that reaction? Especially since they already know what we already know, that he wants to be the King of Pirates. Which naturally Oda wouldn't have hidden anyway.
So Luffy said something else...
I often say One Piece is a celebration of the cliches of shonen/adventure stories, one that follows them and executes them well. But there are things about it that are very abnormal.
And just how mysterious Luffy actually is to us the reader is a big one. This isn't in a Goku way where he has a secret backstory...but Goku is as clueless to this as the reader. Luffy knows things we don't that just pop up later in the story and shock us (Ace in Alabasta, Garp in Water7). And here our main character declaring I guess a more detailed specific form of his dreams...and we don't get to see it.
What the heck did that boy say.