One piece is actually the future? How come? I will explain why I think that in this text.
Did you ever have a history class? But in that class did you learn about the medieval age? Well actually the medieval age is a period of history just after the antiquity where all technology decreased. The colosseum is more modern than any stadium we ever built. I think that must of happened with our age because most of the technology is more advanced and less advanced in some way. Like the thousand sunny, it has so much technology in it but it is just made of wood! And the mermaids, they must have evolved from a fish like humans in less time than us and that is why they still have their fish appearance (yes we are descendants of fishes). And maybe the devil fruits are scientists that genetically modified it so the person who uses it gets a mutation, and that is the only surviving thing about our history. And the void history is actually the history we know right now, and maybe our humans found a super hard rock a mix of plenty of them to write on so it would not break. Maybe the secret weapon is a metal warship or a nuclear bomb in some sorte.
Thank you if you read this theory to the end and share it with your friends, up vote it or leave a comment on what you think.
If this theory gets over 50 up votes I will explain it with more details in a YouTube video here is my channel: Fanly Oko (I didn't do a lot of videos).