It’s notably something they want “to be.” Does that mean it’s something they want “to be the one to do”, or is it some sort of personal status- “I will be happy/sad/free“ etc.
The reactions of other characters are also interesting. It’s meant to be something really childish that people will laugh at.
Ace expects it to be something that Yamato would laugh at, Whitebeard outright roars with laughter and says Roger is a little boy, Sabo finds it funny.
The reactions we have from other characters are the best clue we have:
-Ace is dumbfounded
-Sabo finds it funny
-Whitebeard finds it hysterical
-Oden is amazed
-Yamato is brought to tears
Also interesting is that Roger says he won’t achieve it until he gets to the end of the grand line. Luffy does have to become PK to achieve it.
I honestly don‘t know. Like, a childish thing to say would be something like “I’ll be king of the world“, but that doesn’t fit Luffy or Roger at all. I don’t think a child would be expected to talk about concepts like freedom or something like that, so that wouldn’t fit. Something to do with making the whole world laugh, bringing Joy like Joy Boy?
