Upon asking Oda whether the ending of One Piece was already determined, Oda answered:

Eiichiro Oda: “Yes, I already decided on how it’s going to end. Should I tell you now?”

Momoko Sakura: “Well, I want to know, but I probably shouldn’t. But, the One Piece, it’s not something like “the growth your heart went through”, right?”

Eiichiro Oda: “Hahaha, no no, I won’t be pulling off some kind of Wizard of Oz thing like that. After having gone through an adventure like that, it wouldn’t be fair to not actually give them a reward.”

Then we could deduce that the One Piece ISN’T just “a dream” or “the bonds created among the straw hat crew” or “the journey itself” but it’s a physical reward.