Even though Oda said he only cares about people talking about the "true ending" of One Piece, he still cares too much about people figuring out things that are close to the end of the series. There is no way Oda reads this forum, but since editors might through hits/searches I guess there is some risk with posting everything I've pieced together about the stuff right before the end of the series.
For organization purposes, I'll have things categorized by spoiler tags to skip ahead as desired.
So maybe contradictory that I'm going to start with the biggest stuff, but there's already a whole bunch of theories on the Moon Residents from Enel's cover story, so I think we should all agree that the residents of the Ancient Kingdom are the Moon Residents that had futuristic technology potentially thousands of years ago. The key is that they successfully integrated into the Earth residents and established a country that began to thrive based on the super-advanced technology.
Context implies D.s are the descendants of Ancient Kingdom residents, but the juicy theorization is why they are so few and why they are spread all over the world. Some people may overlook the confirmation that a Pluton DOES exist and is hidden in Alabasta.
In addition to what Robin read and all the intel Croc had, the context of the Void Century and the history of the Nefatarri family implies a Pluton has already been created. How does an army with technology essentially 1000 years old defeat an army with technology that is thousands (emphasis on the plural) of years ahead? Especially since the Ancient Kingdom were open minded and probably had many elite strategists, the easiest answer is using their best weapon against them.
Since they are peace-lovers, the Ancient Kingdom had no intention of using Pluton, but had to build it just in case. The original 20 countries successfully steal it and use it against the Ancient Kingdom to win the war and thus ending the Void Century.
The results of this explain the geography (that doesn't seem like the right word, but I'm tired/hungry) of the New World/Grand Line as a whole. The Ancient Kingdom used to be ALL of the Grand Line islands together. Laugh Tale is the capitol of this huge island (Elbaf, which is HUGE, is the biggest chunk of this; I'll come back and add the official picture later => interesting side not is that it is the only island, so far, that crosses the Calm Belt. Because it is so dangerous for most fighters, it isn't used to cross into the New World).
This explains why only Grand Line islands have a magnetic field, and more importantly why the Road Poneglyphs have to be used to find Laugh Tale (since it's the reference point for all the other magnetic fields). I guess mini-theory is that all of the advanced electronics is what created it, but I guess there could be something "magical" about the island (like God Valley, which is implied to be in the New World).
Since Poseidon literally can destroy the world (destroyed a big chunk of it in probably one shot), the blast is more than strong enough to kill even powerhouse fighters with mastered defensive busoushoku. Since the shockwaves are strong enough to kill normal people, this explains why so few D. Clan individuals survived.
A big part of the Ancient Kingdom is their diversity. Oda shows the normal flow of genetics/evolution in all aspects. Thus, the genetic diversity among D.’s means that they integrated and married outsiders for generations and generations (likely 100’s of years). Which also means it took a while for the hatred/jealously to develop.
So we pretty much know the gist of the true “history”, but this aspect focuses on the creation of poneglyphs. All it takes is just one samurai (with excellent craftsmanship) to travel to the Ancient Kingdom. However, who this was is key. We know it is a Kouzuki and that the prosperity allows for expansion of the family clan so that some return to Wano Kuni. However, the big twist is probably with Toki.
This is a WHOLE separate theory: I’m really thinking Toki is an Ancient Kingdom resident…so she marries her exponentially distant relative (distant enough to not have any negative genetic passing to Momo/Hiyori). The key breadcrumb is that Toki knows parts of the Void Century history and is purposefully looking for Kouzuki in Wano Kuni.
I assume it is more just for personal reasons and wanting to confirm that her country’s efforts, family’s efforts, and personal hardships were not in vain. Since I don’t think she’s dead, she will be a major key when Luffy does succeed in overthrowing the Government.
I don’t think I can top this last part, so I’m probably missing at least some stuff, but I’ll end the theory here.