With a world as large and complex as One Piece's, there are tons of mysteries that fuel the fire that are fan theories. Sites like YouTube and Reddit are teeming with these theories and mountains of evidence to seemingly back them up. One of the major theories that has been making rounds is what the national treasure of Marijoa might be.
YouTuber Joy_Boy Theories is one of many One Piece theory channels and, in November of 2016, they made a video about what they believe the national treasure of the World Government is. According to them, the World Government has a tree that spawns Devil Fruit. This would explain how so many admirals in the Navy have such powerful abilities. There is a tree hidden within Marijoa that produces Devil Fruit that are then distributed to those loyal to the ruling powers. Since the World Nobles wouldn't know what the abilities of these fruits are when they are first created, they test them out on slaves before killing them to take back the Devil Fruit.
There is a drawback to this theory in that, when a Devil Fruit user is killed, the fruit they ate doesn't respawn on the tree it came from, but instead spawns in a nearby fruit. This means that if a Devil Fruit user dies away from Marijoa, the World Nobles can't get it back unless someone working for them finds it first. This explains why pirates and other people have been able to find the Fruits and have powers of their own.
Back during the Dressrossa Arc, Donquixote Doflamingo explained to Trafalgar Law that he sought to steal his Devil Fruit powers specifically for the Immortality Operation and Personality-Switching Surgery abilities that the Op-Op Fruit possesses. With those powers, Doflamingo would be able to infiltrate the World Government and take the national treasure, becoming an unstoppable force that could take over the world. Joy_Boy theorizes that with the Personality-Switching ability, it would be easier to sneak into Marijoa without much pushback because he could just implant his personality or that of someone who works for him into a person from the World Government.
His original plan was to have Corazón eat the Law's Devil Fruit and use the Immortality Operation on Doflamingo. This would have killed Corazón, as the user dies upon using this specific ability, but Doflamingo would have become immortal. The theory then states that with this new immortality, Doflamingo would enter the area with the Devil Fruit Tree after switching personalities, and then eat all of the Devil Fruit available. Because normally it's not possible for someone to eat and use more than one Devil Fruit -- with the notable exception of Blackbeard -- it's thought that by undergoing the Immortality Operation, Doflamingo would then be able to gain the powers of multiple Devil Fruit.
There are other theories as to what the national treasure is. It could even be the Uranus Ancient Weapon. So far, there has been little to no information on what Uranus is or where it might be located, but given how powerful the Ancient Weapons are supposed to be, it would make sense that the World Government would lock one away were they to find it. Shirahoshi the Mermaid embodies the Poseidon Weapon, as she has the ability to speak to the Sea Kings, so it's also possible that Uranus is a living being. If Doflamingo were to infiltrate the World Government and gain control of this weapon, he would indeed be able to take over the world.