English-speaking fans may not realize that sometimes, Oda has characters in the know refer to One Piece as "ひとつなぎの大秘宝" while having the furigana for “One Piece” written above.
For example, Whitebeard uses it when referring to One Piece in his final words.
This, in essence, gives the phrase “One Piece” multiple meanings in Japanese. Moreover, "ひとつなぎの大秘宝" can be translated a few different ways and may be the key to understanding what One Piece is.
"ひとつなぎの大秘宝" - “One Incredible Hidden Treasure”
"ひとつなぎの大秘宝" - “The Incredible Hidden Treasure that Connects People”
"ひとつなぎの大秘宝" - “The Human Jumpsuit Hidden Treasure”
"ひとつなぎの大秘宝" - “The Hidden Treasure of One Calm Sea”
Speculate away, but this context should be helpful to a lot of you theorists out there.
Edit: It may also be worth digging into homophones like “Won Peace.”