My theory regarding Uranus..
The third ancient weapon so called Uranus is the DFs, it was created long long time ago to counter pluton and poseidon. Poseidon is the natural gift of controling sea king while pluton is super weapon hiden inside the moon.
So in ancient op verse there are 3 powerful races; human, mermaid/merman/fishman/, and people's with wings. There was world war that happened in the void century which was the war between these three races. The wing's people lost and went to the moon while the fishfolks went to fishman island, the human with their army armed with DFs won the battle and formed the WG with celestial dragon as their top leaders.
So how about one piece ?
one piece is the last tablet of ponelegyph which contains true history of wht happens during void century where merfolks and wing's people been slaughtered by human. Roger who can somehow read this stone want to over throw the government but he cant due to his illness, so he went to southblue to meet Ace's mom and got married in the hope his child will continue his 'mission'. To protect his child and to inspire the world to chase the dream and freedom he gave im self to the marine and seconds before his excecution he set fire to the world with his words and start the pirate era to increase the probability of someone like him will arise and turn the world upside down (something which he cant achieve).
So yes there will be second world war which will involve the three great weapons; uranus (DFs users), poseidon (seaking), and pluton (great machine controled by Enel).
DF isnt natural magic fruit, it is a weapon created by human and thats why Vegapunk be able to decode the fruit and tranfered its power to objects and his robot.
ok I'm done, this is a theory so dont judge for being rite or wrong coz honestly I dont care