So, this seems to make sense, it's so far just the basic things regarding Void Century things we already know, newest detail we know is he's the first pirate.
Mother Flame seems to check out as energy source, especially since it's a flame.
And the reason the world will "sink" is supposedly due to "manufactured earthquakes" which ties in with the destruction of Lulusia which raised sea levels due to the earthquakes that came as a consequence.
If more Islands get destroyed, sea levels will go up more due to more earthquakes.
I'm guessing the WG is going to use their power with the Ancient Weapon or w/e destroyed Lulusia to hold the world ransom, otherwise the world will risk being flooded.
Hypothetically, if the WG own an Ancient Weapon, they can request countries become a part of the WG by force and pay the heavenly tributes. Refusal to do so means they get destroyed, but being destroyed is not just that nation's problem, it's everyone's problem.
If a neighbouring country is happily paying tributes, and my nation doesn't want to pay, if my nation is destroyed, the country paying risks being destroyed by the earthquakes and sea-levels.
Glad to se Joker is mentioned, and I suspect he will be freed somehow.