BB can be this third party
He doesn't fit though, He is a D. and part of Davy Clan,
He wants to join WG regardless if He betrays them or not,
He isn't yet Strong enough to beat entire World,
He is Part of the War,
This Third Party was Top 1 since Chapter 1, He is just a Spectator, not Player,
I think Narrator is better candidate,
However, there is a Clue to discover who He is,
Crocodile stole Water, Enel stole Gold, Kaidou stripped Samurai of their Swords,
These Symbolize their Weaknesses, what They don't want to be used against them, even though their Powers Counter such things,
So FV should have stolen something from The World which He Counters but at same time can be used against him, so how do we figure out what the World is missing? We ask a Lonely Man,
Sky Island Saga started with Cricket and his Little Gold right?
Arabasta started with Toto and his Little Water right?
Wano started with Tenguyama and his few Swords?
So if I'm right, then in Start of Story, Luffy should have met an Isolated Old Man who fits such Role,
And Yes, We did, it was Gaimon, and what was his Treasure? Rare Animals (Hybrids) which is a concept that Oda finally brought back in this Arc/Saga,
I told you before Chapter 1150 was Arrival of Imu (King of Science),
Chapter 1200 will be Arrival of this Villain who is King of Nature, He is Kars of One Piece,
Ultimate Buccaneer.