Yeah, I think the event involving the original Vander Decken makes sense.
The current Vander Decken is the Ninth. If we assume every generation is seperated by 20 to 30 years, with a average of 25 that means 9 generations ago would have been around 225 years ago. So, I think that puts the original Vander Decken in the correct time period to be responsible for all this.
So, what else do we know? Well, Emet woke up, so that means he must have heard the Drums of Liberation to recharge him. The Fighting Fish destroyed the bridge to Green Bit, hinting that something made them go crazy. The Fishmen attended their first and only Reverie, so something went wrong at that big gathering. The Arms Shop is built in Loguetown. And the St Briss heads to the Grand Line, eventually ending up on a Sky Island.
So here's what I think might have happened.
200 years ago, the Briss Kingdom decided to try and force the reunion of the Ancient Weapons before the prophesied time. Sending out different teams to different areas around the world in order to gather all the keys together in order to overthrow the World Government earlier than planned. "Briss" could also be translated as "Bliss" or maybe even "Bless" or "Barista". So, maybe the Briss Kingdom actually worshiped Joyboy, and wanted to see his plan fulfilled as soon as possible? Even though, trying to complete Joyboy's plan early was probably a VERY bad idea.
The St Briss was sent to the Grand Line to find a way to Skypiea in order to find Uranos. It's also worth noting that Saint-Bris, MIGHT be a reference to a type of wine. Kind of fitting if the ship was meant to help Joyboy "deliver Bink's Sake".
https://en.wikipedia.org/wiki/Saint-Bris_AOC
Vander Decken the First was sent to Fishman Island to recruit the current Poseidon.
I'm not sure about Pluton. Maybe Ryuma was hired to go to Wano to find Pluton? Maybe someone went to Water 7 to try and steal the Pluton blueprints from Galley-La? Maybe the Galleila Giants had to go on the run in order to avoid being hunted for the Pluton Blueprints, which eventually lead them to hiding them with a group of humans on Water 7, which eventually became Galley-La?
And chances are someone was sent to track down the Nika Fruit, which might have been Joyboy's originally. That could also have been Vander Decken the First, but it could have also been someone else.
The Arms Shop was probably built in Loguetown as a front. It might have been meant to secretly find ways to arm the civilians of the town, in order to foster a revolution. Or maybe whoever started the Arms Shop was originally trying to find and hide away some legendary sword?
All these plans probably went terribly wrong.
The St Briss not only seemingly failed in it's mission to retrieve Uranos from the Sky Islands, but it's also possible that their foolish actions directly lead to Uranos falling into the hands of the World Government. If the World Government is indeed using Uranos as the weapon that destroyed Lulusia. The crew of the St Briss died by being punished by Cloud Drifting. But, it's possible a few members of the crew survived by piloting Uranos out of there.
(Could this also have something to do with the Oykot Kingdom and Nami? Maybe when the Briss Kingdom stole Uranos, they also displaced the "Shrine Maiden" for the device. And Nami's ancestors were forced to retreat to the Lower World, in order to save themselves? Or fell to the surface world, assumed dead by their people? Since Nami does have a lot of connections to weather, it wouldn't be impossible for her to end up being the User of Uranos, much like Shirahoshi is for Poseidon, or Momo seems to be for Pluton. It's a stretch, but I'm still expecting Nami to get some sort of backstory.)
Vander Decken the First probably tried to FORCE the current Poseidon to awaken, but failed. Probably killing that Poseidon-candidate in the process and sending Fish and Sea Kings around the World into a frenzy. Reacting to the psychic pain of losing someone they had a connection with. All these Fish and Sea Kings going out of control around the World was blamed on the Fishmen. All this probably happened DURING the Reverie that year. Thus, it would appear to the World Government that the Fishmen were trying to command the Fish to attack and overthrow the World Government, even though the Fishmen had nothing to do with any of this. Esepcially after Emet wakes up at the same time. Thus, the World Government forbids the Fishmen from joining the World Government, and the discrimination against the Fishmen reaches a all time high.
Someone with the Nika Fruit was likely also either at the Reverie that year, or at Fishmen Island at the same time that Vander Decken the First tried to force Poseidon's early awakening. We know that Emet has to be relatively close by in order to hear the Drums of Liberation. And Fishman Island is right below Mariejois, so the Nika User could have been at either location. It's possible that 200 years ago, a Nika User and a Poseidon User both met, but before either had awakened their powers. This might be what the Sea Kings were referring to with the line about "surely this time it will work out". (Chapter 648, Chapter 967-968). It's also possible that Vander Decken the First tried to force-feed the Nika Fruit to Poseidon in order to awaken both at the same time. That MIGHT be what killed the Poseidon-user. Either way, the Drums of Liberation were silenced relatively quickly, meaning the Nika Fruit User likely died around this event.
Pluton, again, is a bit of a mystery. It's possible that the people chasing after the Galleila Giants eventually captured them on Punk Hazard. Where they were frozen. Possibly because Pluton has some sort of ice beam. So, it's possible the Giants froze themselves, rather than let themselves be captured and forced to reveal Pluton's secrets. Unable to thaw them out, this mission also ends in failure.
The Arms Shop in Loguetown therefore never gets the call to start the revolution against the World Government. So, it just continues onward as a regular sword shop for hundreds of years.
And the mastermind behind this attempted coup may actually be...The Shephard Family. The Shephard Family is likely one of the 20 Founding Families, but we don't know for sure which land they originally ruled. It's possible that the Shephard Family were the rulers of the Briss Kingdom. The St Briss Ship has the same symbol as the mark on Saint Sommer's arm. That might not be a "Abyss Mark" but instead the "Nika-Sun-Ally-Symbol" for the Briss Kingdom. A family crest, as it were. Like the ones for Amazon Lily, Wano, and Alabasta. And Saint Shephard Ju Peter has been acting odd. Almost like he could be a traitor. Maybe he was, 200 years ago? Maybe the Shephard Family were the ones who were trying to reunite all the Ancient Weapons together 200 years ago? And it would kind of make sense for the "Shephard" Family to try and lead the world to their God. "Shepharding" the world to Nika's light.
That may also explain how the World Government got their hands on Uranos. Maybe Uranos was the only one of the weapons the Shephard Family managed to capture. But, in order to hide their true intentions, they had to hand it over to the World Government, claiming it was all part of a plan to capture one of the Ancient Weapons for Imu. Ju Peter would still have access to the Weapon. Maybe he thought that, even in the World Government's hands, he could find a way to steal it back it later? Maybe that's why Ju Peter was acting so strange about the Nika Fruit being renamed as the Gum-Gum Fruit (Chapter 1037). Maybe Ju Peter is actually the one who renamed the Fruit in their records in order to keep anyone else from finding it and he was trying to cover his tracks...poorly. ("How else could you possibly explain it?!")
This would make this one of the biggest failed conspiracies in the One Piece World. One of the biggest blunders in history, that only helped to ruin Joyboy's 800 year plan, rather than aid in achieving it.
I'm not sure how likely this all is. But, I could see Oda doing something like this. The Shephard Family tried to reunite the Ancient Weapons early, and it all went terribly wrong. I could see us getting a flashback about that, if and when we learn more about the individual members of the Gorosei. If I'm on the right track about any of this.
