@KageKageKing:
Because certainly the world's largest political organizations will be so vulnerable in there.
Right.
It's not that Luffy and Jinbe need to be there for diplomatic relations or as a typical body guard. As a member nation of the World Government they should be fine. I'm considering the possibility that Shirahoshi being revealed as an ancient weapon throws everything into chaos. The government soldiers and Marines who are there to keep the peace may be ordered to seize her or the government tries to talk things out and work with the Neptunes to keep things in check on Fishman Island, all the while the Government eventually gets almost complete control of who and what goes in and out of the New World, above and below. Thats fine as far as our heroes and Fishman Island is concerned. But then Kaidou finds out about Shirahoshi through Caribou so why wouldn't he go after an ancient weapon that controls the mightiest beasts in the world?
There could be a war for Fishman Island and that is where Luffy and the rest come into play. Kaidou could send a few forces right up to Mariejois in hopes of seizing Shirahoshi, Luffy and the rest get wind of it and decide to intercept. The battle could take place right up in Mariejois, in heaven where the Eve Tree grows and Luffy completely smashes it to bits on purpose to keep Fishman Island completely uninhabitable so no one gets such a strategic base of operations.
My theory has nothing to do with Luffy helping Otohime's dream the way she intends. Indirectly it helps by forcefully pushing people into Noah for a mass migration but the diplomatic portion of that wouldn't come from Luffy or Jinbe.
I think we are near guaranteed a full out defeat or even the death of Kaidou by the end of this coming Emperor War. An important piece has already been in play for a very long time regarding Shirahoshi and Kaidou. I have to assume Oda is going to play that piece and move it forward. Kaidou learning about Shirahoshi now and knowing where she will be just seems to line up too perfectly. I feel like Kaidou leveraging that intel would work best long before his defeat.