In the most recent chapter, we saw this panel of Caribou talking about how a “certain someone” would love to hear about not only Pluton’s existence in Wano, but also the relation between Luffy and Poseidon.
I’ve seen a handful of theories and discussion trying to figure out who he’s referring to. I think that the most probable answer points to a character that seems to have been largely forgotten: Shiki.
Shiki is, to our knowledge, still alive, and simply hasn’t been active in the story. Looking back at his canon history:
Twenty-seven years ago, two years before Gol D. Roger and his crew reached Laugh Tale, Shiki had learned that Roger had located an ancient weapon. Seeing the opportunity, he asked Roger if they could join forces. With his large pirate fleet and Roger’s knowledge of an ancient weapon, Shiki believed that together they could take over the whole world and asked Roger to become his right hand.
From this, we learn that Shiki’s goal was to control/take over the world using the power of the Ancient Weapons (or at least one of them). This concludes with Edd War, Shiki being imprisoned and breaking out of Impel Down, and eventually onto an unexplored island, where the wiki mentions the following:
He asked Dr. Indigo how the experiment was coming and was told by Indigo that he had made a discovery that there was a balance between the plants and animals on the island. He also explained that many plants and animals could not be found elsewhere to which Shiki interrupted him. Shiki ordered him to get straight to the point, and asked Indigo how long would his plan take, and was told that it would take twenty years at least. Accepting this, Shiki said that he would then carry out his plan in 20 years, and then he would show “them” hell on Earth by releasing tons of mutated animals, declaring war to the world.
The events of Chapter 0 take place ~20 years before the current storyline. We know that a war is coming, and it’s been hinted at throughout the entire series. Kaido wants to prepare for the war, Whitebeard told Sengoku that he was afraid of the war that would one day embroil the world, etc. Shiki is someone who actively wanted to wreck havoc on/gain control of the world, and planned to use the Ancient Weapons to do so.
I believe that Caribou is somehow associated with him, and plans to report the information that Luffy is now “in possession” of two Ancient Weapons back to Shiki. He will eventually go after either the weapons, Luffy himself, or both in a second attempt at the same plan that he offered to Roger, this time with… mutated animals too? I’m sure that piece will fit in somewhere, but the bottom line is that Shiki vowed to declare war on the world 20 years from that moment, and now, 20 years in the future, with foreshadowing of a massive war that encompasses the world, a “certain man” would be interested in the weapons. Out of every character that we know the motives and goals of, Shiki seems to be the most reasonable guess.
*Theory by 1Darude1