The world of One Piece is filled with mysteries, especially those concerning the World Government and the Void Century. The very first hint fans got of this was the Alabasta arc, where the Poneglyphs and the idea of the Void Century were introduced. Ever since then, the story has slowly been diving into deeper mysteries. The Skypiea arc told the fans about the Great War that Shandora fought in, the Water 7 arc introduced an Ancient Weapon, and the Marineford arc declared the coming of a massive war in the future.
After the timeskip, this overarching plot surrounding the Void Century, the World Government and the Ancient Weapons continued as Fishman Island introduced yet another Ancient Weapon in Poseidon, and shortly afterward, the Dressrosa arc brought a mention of what could be the biggest turning point in the entire story so far; the National Treasure of Mary Geoise.
What We Know About Mary Geoise's National Treasure
The National Treasure of Mary Geoise was first introduced by Donquixote Doflamingo during his fight against Trafalgar Law in Chapter 761 of the One Piece manga. When Law asked Doflamingo how a former Celestial Dragon such as himself was able to mobilize the CP0 and had the World Government on strings, Doflamingo told him that he knew about the existence of the treasure was hidden at Mary Geoise. According to him, this treasure is great enough to be considered as a National Treasure, something that very rarely comes up in One Piece. In Wano Country, Shimotsuki Ryuma's sword, Shusui was considered to be a National Treasure. At the same time, the Tamatebako held on Fishman Island for generations was also considered as a National Treasure. Simply put, whatever is held inside Mary Geoise is of great value and power to the World Government.
Doflamingo continued that the only reason he wields as much power as he does is that he knows what the National Treasure of Mary Geoise is. The mere existence of this treasure is described to be enough to shake the entire world to its very core, which just goes to show how powerful the treasure itself must be. The World Government tried their best to kill Doflamingo but eventually realized that they couldn't do it due to the ace that he had up his sleeve. Consequently, they listened to every demand of his and gave him power that surpassed that of any other pirate on the seas. According to Kuzan, this made Doflamingo an exceedingly rare class of pirate, even rarer than the Pirate Empress of the Kuja, Boa Hancock.
According to Doflamingo, to make full use of the National Treasure of Mary Geoise, he needed the Ope Ope no Mi one a particular day, and had things gone his way, he would've obtained the greatest power in the world and ruled supreme. This clearly implies that the treasure hidden within Marie Geoise is enough to control the entire world somehow.
What Could The National Treasure Be?
While it isn't known what exactly the National Treasure of Mary Geoise is, Doflamingo did leave more than just a few hints behind and most of them have to do with the powers of the Ultimate Devil Fruit, the Ope Ope no Mi. According to what Doflamingo said, two powers of the Ope Ope no Mi, in particular, would be helpful in making use of the National Treasure of Mary Geoise. The first ability is the Personality-Switching Surgery that it allows the user to perform.
Fans saw this ability being used in the Punk Hazard arc with the Straw Hat Pirates and the G-5 Marines. The other ability is even more important and is what gives the Ope Ope no Mi the title of the "Ultimate Devil Fruit" in the first place — the Perennial Youth Surgery. This surgery allows the user of the Ope Ope no Mi to grant immortality to anyone in exchange for their life. Evidently, whatever is hidden in Mary Geoise can only be made use of with these powers.
One of the most prevalent ideas related to the National Treasure of Mary Geoise is that within the Holy Land lies the Tree of Devil Fruits, Eve. One Piece has already introduced fans to quite a few magical trees, the first of which is Adam, which is located somewhere in the Grand Line. Adam wood is used to make some of the most durable ships in the story and both Gol D. Roger and Luffy's ships are made with it. The second tree introduced in One Piece is the Sunlight Tree Eve, the roots of which absorb the light from the surface and deliver it 10,000 meters under the ocean to Fishman Island.
Curiously, Fishman Island is placed directly underneath the Red Line and Mary Geoise, meaning the tree's top has to be somewhere around Mary Geoise. If true, Eve could very well be the Devil Fruit tree. Using the powers of the Ope Ope no Mi, Doflamingo could've penetrated deep into the heart of Mary Geoise using the Personality-Switching Surgery and since he would've been immortal due to the Perennial Youth Surgery, he could've been able to eat as many Devil Fruits as he wanted and gained absolute power over the world.
Another interesting idea surrounding the National Treasure of Mary Geoise is that Im, who is a mysterious figure ruling the entire world himself is the National Treasure of Mary Geoise, just as Shirahoshi is Poseidon. If Doflamingo had access to Personality-Switching Surgery, he could take control of Im's body and become the next ruler of the world who sits atop the Empty Throne. At the same time, the Immortality Surgery could've been used to make him immortal so he could rule forever.
Of course, it is highly likely that Eiichiro Oda's plans for the National Treasure of Mary Geoise are something much greater and different from what fans have imagined them to be. For now, however, fans can do nothing but speculate. In time, Oda will reveal what rests inside Mary Geoise and why it holds so much significance in the grand scheme of things. Until then, fans need to patiently wait for the story to unfold.