The “Ancient Kingdom” didn’t lost, Mary Geoise was his name and it is now known as the “World Government”.
Here are the evidences :
– First, why don’t the World Government just lie about what happened in the Void Century rather than erasing 100 years of history? After all, if they had to hide something they did in the past they could just say that their enemy was “evil”. It’s what they did with Ohara. History is written by the victors. But here it’s not what they did! Instead of lying about the past, they just erased it. Why Well, to me it’s clear: It’s because they are already lying about it!
– Mary Geoise
When Doflamingo explain to Law how the World Government was founded, he never said that these 20 Kingdoms created Mary Geoise! He said that the royals families created the World Government and moved to it. Mary Geoise was already there before the end of the war. Which mean, it was the capital of their enemy!
When the professor Clover was speaking with the Gorosei, he hadn’t all the clues about the Void Century. So he made an hypothesis. What was the reaction of the Gorosei when hearing it? They said it was an “audacious hypothesis”. But when he was going to reveal the name of the “Ancient Kingdom”, they ordered to kill him. Why not killing him when he said his hypothesis? Because it was just an hypothesis. The name of the “Ancient Kingdom” on the other hand, he was right on it. But then there’s a new question, “Why revealing the name such a big deal?” If it was “Blibidiblu” nobody would care! Well the answer is pretty obvious: That name, everybody know it! Yes, that name is “Mary Geoise”.
– The “Empty” Throne.
Let’s assume that the 20 Kingdoms created Mary Geoise and Pangaea castle. Why would they need a throne to begin with? If they wanted to say “We, the Tenryubito, are all equals, the others are peasants.” there would be no need for a throne. Unless… the throne was already there before they became Tenryubito !
As for the swords at the feet of it, it’s not for a pledge. In the past when a war ended, the one to surrender offered symbolically his sword to the victor! It was a symbol of “handing over the weapons”. Which mean that these are not swords for pledge, but symbols of the defeat of the 20 Kings! And furthermore, each time a new king put a sword at the base of the throne he symbolically express his defeat against the World Government.
In that theory, the Ancient Kingdom being now the World Government… it explain the existence of Im. The Gorosei don’t want people to know about it, because otherwise they would start to want his/her place just like Sterry when he saw the throne! As for the nature of Im, an immortal being? Something like that maybe. To be the “King/Queen of the world” (s)he must have a tremendous Conqueror’s Haki. Or maybe something else that explain why even the Gorosei follow his/her orders. Hey, Im may even be Uranus in person!
– The Tenryubito
If you wonder, why did Im had 19 royals families come to live in Mary Geoise after their defeat? It’s simple: to have them work for him/her. That is the best way to “use” them! Plus, the grudge for all of the bad things that Im want to be done is redirected to them! For Im, the Tenryubito are a shield. But there was one family who didn’t accept: The Nefertari.
They don’t seem really pleased to bow before Im-sama don’t you think ? Well, they don’t have the choice I believe.
The World Government made up the story that 20 Kingdoms formed it when in fact, it was a kingdom and is still one. Over the centuries, the Tenryubito became arrogant by living in Mary Geoise forgetting that it isn’t really them who rule the world. It’s Im-sama.
*Theory by _Zokai_