So it most likely means that the rulers or kings of the great Ancient Kingdom was in fact the D. Family.
They are a family of royalty, and could possibly be the ones that ruled the entire world. But many smaller countries were tired of them controlling everything and so they decided to revolt and destroy them completely (if they weren’t so powerful and had such big influence on the world, then the World Government had no reason to destroy them, they should have allied with them, but because they ruled the world, they needed to destroy them so they can take the D.’s throne).
And the reason the World Government, after defeating them, established their existence was so they can take their place as the rulers of the world. And they wiped out their existence from history because the Ancient Kingdom is the true World Nobles and Kings of the world, and if that knowledge was known, the Celestial Dragons will no longer be respected but rather everyone would want to kill them to make room for the D. Family.
That’s where Dragon comes in, Dragon is, as we know, a D. He is considered the most dangerous criminal in the world and is feared by the World Government just how Roger was. We can tell by everyone’s reaction in Marinefold when discovering Luffy’s father was Dragon.
So maybe, Dragon just like the Roger Pirates, has learned the true history of the world, since it wouldn’t be hard to believe that someone in those 800 years after the Void Century was able to learn the truth other than Roger. That’s why Dragon is more notorious than Whitebeard himself who was Roger’s rival and was just as strong as him. The only reason Roger was more famous and feared was only because the knowledge Roger had about the true history of the world. And that also might be the case with Dragon, and that’s why he’s just as notorious as Roger was and even more so than Whitebeard.
So Dragon realized how greedy and evil the World Government truly is and how they overtook his ancestors home and throne. Therefore he’s creating his own Revolutionary Army just as they did in the past, to take back his heritage. And that’s the reason behind Dragon’s army, to take back the world from the World Government, since they took it from the D. Family and have been abusing their power. And that’s what’s behind his “Dangerous Ideals” and is why he’s the Government’s most notorious criminal. And that also makes the famous line “D. Will Cause A Storm Again” Dragon himself is trying to be that storm himself.
*Theory by Wasabaki