We only know a few things about the Ancient Kingdom, but I think we can paint a picture based on what we know. For starters, the closest approximation of an image looks like this.
This happens to look like an ancient Greek/Roman city, the pillars are a dead giveaway, and it’s honestly a little suspicious we would be given a tease like this when very few things in the series have Greek/Roman ties, but the ones that do speak volumes.
For starters, we have the Ancient Weapons, Poseidon, Uranus, and Pluton named after A Greek God, A Greek Titan, and a Roman God, respectively. We have Lunarians, specifically King, who seem to have a tattoo inspired by a laurel wreath.
Lastly, the Ancient Kingdom and The Romans share similar backstories. They were once the seat of a massive empire that held much control and influence over the world, and then they fell to ruin which paved the way for the world we see today. (credit to Hidden Island for pointing some of these connections out). Rome also had a history of Slavery, and we know slavery existed as far back as the ancient kingdom as well.
So, it seems like the Ancient Kingdom has heavy Greek/Roman influence and the Ancient Kingdom itself was the One Piece equivalent of Rome, just like how Dressrosa is the equivalent of Spain, Wano is Japan, and Alabasta is Egypt. We know the name of the Kingdom has to be a recognizable word or name in the series, otherwise they wouldn’t have shot Clover for almost revealing it, because the very word itself is reprehensible to the World Goverment.
I think we can determine the name of the ancient kingdom, as it’s a word that itself is a major theme of the series, and happens to literally come from the word “Rome”.
The Ancient Kingdom of ROMANCE, or Romance Dawn
I think the name of the Ancient Kingdom IS the name given to us in the first chapter of the series, and was almost the name of the series itself before Oda called it “One Piece”. Romance comes from Latin Romanicus “of or in the Roman style,” from Romanus “Roman”.
We know the word “Dawn” is already a huge aspect of the series, it’s straight up an event people are waiting for, and in cases like Pedro and Kiku, willing to give up their lives for.
At the end of the day, this is still a Pirate story, so while the name and design of the ancient kingdom might come from Roman influence, I think it’s history will be something more rooted in Pirate history/mythology. Specifically, the lost Pirate city of Libertalia.
Libertalia was a allegedly a legendary free colony founded by pirates where society would be one in which people of all colours, creeds, and beliefs were to be free of any scrutiny. Sound familiar, because that sounds a lot like what Luffy’s journey itself is building to, and what the government itself has worked so hard to prevent.
I think the Ancient Kingdom, Romance, was exactly this, a tribe where people of all races could one day come together and share drinks and sake like brothers, waiting for one day where their Kingdoms promise would be fulfilled, waiting for a day called….. The Romance Dawn.
Theory by Ocean Man (https://www.reddit.com/user/Clean_Strategy_9533)