Like Rosco said: in the absence of wrongdoing there would be no reason to hide history. Considering the lengths they will go to suppress the true history, it can be assumed that knowledge alone is enough for the World Government to lose legitimacy. The liberties given to the Tenryuubito coupled with the suppression of history implies the Twenty Kings were some kind of tyrants, violently overthrowing the ancient kingdom.
My guess is that the ancient kingdom was a utopia of sorts, a place of equality and prosperity. Chances are the Twenty Kings were local rulers desiring absolute power. To do so they formed an alliance to overthrow the ancient kingdom and seize power for themselves.
One might guess, that the need to annihilate whole tribes and islands, like Robin's Oahana archeology clan, is an attempt to hide the revelation of the cause and reason of the assumption of power. This concludes that the reason would cast a slur at them because it was to crave power.
Yeah, it's why I think the 20 Kings were bad. However, they may not have been as bad as the current Tenryuubito since I recall someone saying that as the generation passed, the descendants got worse and worse in behavior and personality. Doesn't necessarily mean The 20 Kings weren't evil (aren't they founders, rather?), but they may not have been as bad as the Tenryuubito.
The Tenryuubito may have become more decadent as the years passed, less inclined to rule, more interested in creature comforts. The Twenty Kings may have actually taken an interest in more than luxury.
Neptune knows at least a little. He knows that Shirohoshi is one of the ancient weapons, and he knows what is recorded on Fishman Island's ponyglyph. Don't forget that knowledge is suppressed and keeping quiet is in his best interest. Neptune does not seem to know the complete history, only Roger seems to know that much, but he does know something. The Blank Century was 800 years before the current time line, the government has suppressed knowledge by force. Very few remaining people seem to know even a part of the history.
That is my guess as well, the three weapons were intended to challenge the Twenty Kings. They may have been intended to prevent the Kings from deposing the ancient kingdom, or they could have been created after, as a means to revolt, when the time is right. Joy Boy's message makes me think that the weapons were never actually used in battle.
Hmm, the first D. could have been the leader or prominent part of the Twenty Kings who overthrew the kingdom/s and set up a government, but he was the most honest or the most nice. Maybe he was the good one who kept others in line until he died or was murdered. If I recall, Blackbeard is the only exception to the Will of D, someone who killed his own nakama and all.
Could have been that they weren't viewed as supremely elite yet that they didn't act as douchebaggy as their descendants.
D'you think that what little he knows about the blank century has helped him hate humans (holy crap, alliteration! <3)? Though, I'm not confident with the thought that the abuse of fishmen would motivate teh WG to keep the century hidden and unknown considering they still allow fishmen to be sold for slavery.
Could they have been the Twenty Kings' weapons, rather, that were refused to be made or used by them? It looks like something similar to Shirahoshi's power was used before. Now that I think about it, the blank century could also be the most embarrassing period for the WG, lots of mistakes, losin wars, stuff like that.
A D. would be part of the 20 kings, though, not be the sole ruler. Maybe there was something horrible that happened to him and his kingdom that convinced the WG to create blank century. Didn't two D., Luffy and Gol, come from East Blue? Maybe East Blue was a D.'s kingdom.
Dragon, Luffy, and Roger are both from East Blue. Ace was not born in East Blue but obviously he grew up there. Roger is native to Loguetown. In chapter 586 it was confirmed Dragon was from Goa Kingdom, where Luffy grew up. There is definitely a connection between D., the Blank Century, and the Ancient Kingdom. All four of the currently identified D. have strong ties to East Blue. That has to be extremely important. Right now I could not do more than speculate on what the connection is.
