@K.:
At this point is more likely that the Ancient Kingdom was attempting to install a one leadership type rule throughout the world with their advanced technology. And 20 nations band together to fight them. In the process, the 20 nations did horrible things as well and because they were in such a position of power after defeating the kingdom they manipulated information so they could establish a proper rule throughout the world. Which would entail the Void Century.
So Rayleigh was: basically, they all did bad shit.
More or less, although it certainly seems like Im and Co. are attempting to do the same by means of the Tequila Bridge and what not. IM herself ( yes a she imo ) is basically the Queen of the World at this point, but as others theorized, the power vacuum left by the absence of the AK's leadership probably led to an all out 100 years war type power struggle between the various kingdoms similar to the fracturing of Alexander the Great's Empire. The whole Empty Throne/United Nations setup itself was intended to deter any power hungry idiots such as Sabo bros attempting to target and usurp IM. That was the lesson learned in the wake of the fallout of the Ancient Kingdom's demise. Same end goal, but a more devious approach.
My own take: the status quo of today is a reversal of that which existed back in the day, with fledgling humanity occupying the lowest rung of the hierarchical ladder and the non-human races of the world ruling over them, with real actual dragons ruling at the top ( ie Donquixote's ancestor basically fought dragons instead of windmills…or maybe fought windmills while someone else did the fighting for him :V ). Early humans basically got the early Christian/Hebrew treatment before ultimately waging armed rebellion with the freedom-loving D clan at the forefront ( doesn't matter who's in charge ), along with a certain Messianic type who got Yasu'd the fuck up, and a sympathetic Promethean benefactor bearing gifts that aided their efforts. World gets wrecked, reprisals are bloody, and vicious infighting ensues after humans prevail. Im and the Elder Stars conclude that people are fucking stupid and decide to cover up all this shameful history in a futile attempt to avoid a repeat of the past ( spoilers: they're fucking it all up and retreading the same path to failure. ).
So yeah, it'd def throw the world into upheaval if so; Celestial Dragons are revealed to be liars and hypocrites; non-humans learn they used to be on top before getting ousted, that they're suffering for the sins of their ancestors, and oshit some of their ancestors may have been real bloodthirsty assholes to boot; oh hey, human nations, turns out some of your WG allies took advantage and actually stole your land from you in the distant past! Add any revelations about IM, her and the Elder Stars' probable immortality, the existence of the Ancient Weapons, and all hell and pure chaos would be sure to break lose.
Big moral conundrum for Roger and Co. for sure. I'm thinking the foremost worry they had in mind w/ regards to spilling the truth would be the chances of history simply repeating itself, setting off a wave of hatred, violence, and revenge...one big vicious, never-ending cycle with the roles of conqueror and conquered, oppressor and oppressed, etc. changing hands back and forth. Oda's basically been heavily emphasizing the cyclical nature of power and conquest all throughout the series, all culminating in this big reveal imo. Brooke's experiences with that devil-worshipping tribe he freed from slavery in particular really framed it all to me: recall the first thing they decided to do upon gaining freedom? Gleefully stating their intentions to subject their tormentors to the very same treatment.
Those are the broad strokes anyways
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Edit:
Eh, why the fuck not. I'll add that I also suspect reincarnation will factor heavily into the plot later on as well. My theory is that Oda will reveal that the heroes of today are actually the reincarnations of penitent villains from the past, the villains the vengeful spirits of ancient saintly figures...the ultimate Reversal of Roles, karmic reversal of fortune, and thus divine punishment/learning lesson.
The Roger Pirates decision to withhold the truth about the Void Century didn't just come down to a matter of avoiding potentially catastrophic fallout...it also partly came down to Roger and Co. simply not believing themselves worthy of setting the world right since the sad state of the world throughout the centuries can be directly attributed to actions taken by their prior, corrupt incarnations. In other words, Roger and Co. were all incredible assholes in the distant past, Roger in particular probably being the opposite of how he was portrayed in Skypeia in the most recent chapter - a gold-obsessed, King Midas-like hoarder. On that note, I wouldn't be surprised if Past Roger met a similar fate, having once become a true Gold Roger lol. Cue Roger making fun of himself same as Luffy did when the latter got turned into an ice cube by Aokiji. PARRALLELS!
Somehow or another, reaching Raftel allows one to unlock said memories of their past lives or gain access to said hidden history. The One Piece? The last remaining piece of a once-massive treasure horde that Past-Roger accrued in the distant past, now crumbled away to near nothing as most empires tend to do: every dog gets their day in the sun, but few things last forever ( THEMATIC! ). Luckily for the Strawhats, what appears to be some shitty lil nugget of treasure is actually the single greatest "treasure" Roger ever found: a literal wish-granting jewel. It's the Philospher's Stone, the wish-granting Pearl of Eastern Dragons ( DRAGONBALL SHOUT-OUT ! )…but it's true nature is that of a hunk of matter in its most fundamental form in the context of the One Piece world - haki.
Yes, anything and everything in the One Piece universe is composed of materialized haki of a divine sort; manifestations of "God's" will and imagination ( recall the narration when the St. Briss first fell out of the sky during Skypeia arc? ). Everything that exists in One Piece is born from the thoughts of God/the Universe/Sleeping Vishnu/Dreaming Cthulhu/Aslan/whatever the fuck you wanna use...The spirits of the devil fruits are "daemons," living embodiments of ideas and concepts that can be given form, fruits picked from the Tree of Knowledge from the Garden of Eden...Everything in One Piece comes down to imagination and will.
But going back to the One Piece, it's essentially a Divine Blank Check that simply requires the application of one's own haki to basically make one's wish transformed into reality. Why did the Roger Pirates never use it? Probably to avoid some Monkey Paw scenario ( LUFFY! MONKEYS! ). Again, see King Midas.
Also, somewhere out there is a very literal "God" ala Vishnu, comic Lotus ( World Tree, Sephiroth, Treasure Tres, yadda yadda ) in danger of being woken up by a certain somebody ( ironically one unable to sleep? IRONY ), threatening to cease the dream that is all of reality as we know it in the One Piece world.
Blah blah blah that's my head-canon.