Could Pluton possibly be...?
Pluton is said to be a a warship that is capable of destroying whole islands.
What we know now is that...
-Pluton currently exists somewhere and the World Government is trying to locate it.
-Franky used to have the blueprints to creating another Pluton or a countermeasure to the real Pluton.
-There are a total of 3 "Ancient Weapons", and that they are all named after gods.
Now, before we go on, I want to point these two key points out first.
-Pluton is another name for Hades, the ruler of the underworld. In more modern religions, we can often refer that god as the Devil.
-"I once heard that all the Devil Fruits are the Sea Devil's incarnations. If you eat one you'll gain a special ability, but you won't be able to swim." quoted from Red-Haired Shanks to Buggy.
Now, here is my belief on Pluton and the Blueprints Franky possessed.
Pluton is an ancient power capable of destroying islands, but no one knows to what the power entails. They only said it was a warship. However, I believe Pluton is not a warship, but is in fact, the "Original" Devil Fruit. All Devil Fruits are said to be the incarnation of the Sea Devil. My belief is that Pluton is the Sea Devil and each Devil Fruit is created from Pluton and given 1 power that Pluton possesses.
Furthermore, the blueprints that Franky used to possesses might not be blueprints to Pluton, but in fact, a countermeasure to it. What if Pluton were a Devil Fruit that granted you the ability to use any power? What if it fell into the wrong hands? The answer is Franky's blueprints. His blueprints, I believe, must have been a type a prison that can counter all the powers of the Pluton Fruit and enslave the user within.
In short, Pluton is the father to all Devil Fruit powers. But wait... Someone already ate Pluton!
Yes, the 200 blank years was a period of time in which the user of Pluton wreaked havoc on the world and was later enslaved by the very prison in Franky's blueprints. Yes, yes, some of you may be thinking that user must already be dead and Pluton must not exist anymore, plothole in my theory. Well, I shall tell you, I think not!
The answer to that? Yami Yami no Mi. Do you know who possesses that fruit? None other than our dead friend, Brooke!!! It's a fruit that doesn't activate till after you die, allowing you to continue living, a second chance. However, the fruit is meant for you to return to your body right after you die while there is still flesh on it for the sole reason to have an end to the fruit's power by dieing of old age. However, Brooke couldn't find his body and returned years later when the flesh was gone. This means, he can't even die of old age! He's immortal now!
What if the Pluton User is still looking for his body? What if the True history leads the Straw Hats or another unsuspecting victim to the cage and they unlock it? The Spirit of the Pluton user can then return to their body with a second life, and this time, finish off what they started.
What if Gold D. Roger already found Pluton? Back in chapter 507, Rayleigh mentioned that Gold D. Roger and the crew already discovered what happened in the 200 Blank Years and that they made a decision already. What if One Piece was their decision? They left behind the secrets to Pluton, hidden, and only the most powerful of Pirates with the best intentions can find it.
So in short, the final villain is the revived Pluton user that Luffy, the Straw Hats and all those who follow Luffy will have to take down the Sea Devil, the father of all Devil Fruit users, Pluton itself.