In Fishman Island arc, it was revealed by King Neptune that Princess Shirahoshi is the Ancient Weapon Poseidon.
In Chapter 650, when Neptune made the revelation to Nico Robin, Caribou overheard it and attempted to kidnap the princess but failed.
Caribou then had his own cover page mini-arc. At the end of his mini-arc (Covers for Chapters 727-728-729), he was defeated and captured by X-Drake which is currently a subordinate of the emperor Kaido.
In Chapter 795, Kaido exclaimed that he will start a war that will be the biggest war in history.
In Chapter 823, it was shown that the Ryugu Kingdom (Fishman Island) intends to bring along Princess Shirahoshi to the Reverie – the meeting of the World’s political leaders at Mariejois.
Now going back to Caribou, I’m thinking that he will share the information about Poseidon to the Beast Pirates in exchange for his safety since he has been shown to be willing to perform cheap tricks to save his ass. And Kaido (with his crew and allies), in turn, will use this information and attack the Reverie at Mariejois and the attack will escalate into a war since the World Government would probably also want to take an Ancient Weapon for themselves and would also retaliate against Kaido’s attack.
The World Government would use all their powers (Cipher Pol, Marines, Shichibukai, etc.) to attain it. Of course this war wouldn’t just occur in Mariejois.
Ryugu Kingdom could escape from the Red Line in any means possible and call for help while the World Government is fighting a Yonko. Ryugu Kingdom would gather allies such as the Straw Hat Pirates and their allies. News will break out and surely the other Yonkos (especially Blackbeard and Big Mom) and other pirate alliances would also wanna have Poseidon for their own and will hunt Shirahoshi.
I’m sure that there are also political leaders that is a part of the World Government but has selfish nefarious plots of their own (such as Wapol of the Black Drum Kingdom and King Stelly – Sabo’s adopted brother- of Goa Kingdom). Since many innocent lives will be affected, it will be an opportunity for the Revolutionary Army to make their move.
This would start a global-scale war which would involve every political power in the world. Fishman Island could even be caught up in the war and this could be where Madam Shyarly’s prophecy about Luffy bringing destruction to Fishman Island will come true.
And now, Kaido’s big war has been ignited!
Now, I know that Oda has been hinting that the Wano arc will be where the big showdown with Kaido will happen but I think that one arc would not fit a war with one of the Four Emperors and that the Reverie and Wano arc are largely connected since Oda said in an interview back at 2016 that Reverie and Wano arcs will be the next stop after Whole Cake Island. What do you guys think? Is this plausible?
*Theory by retardaniel