Word being destroyed twice has a lot of implication for the endgame
Most assumed the void century was the first thing such event happened.
I firmly believe Joy Boy was from Imu's era, and Nika was from an era before that.
This also tracks with Imu calling Luffy "Joy Boy", not Nika.
If you look at the city of Alubarna, created 4000 years ago, it's clearly built in a manner to deal with heavy flooding:
The high walls + staircases that can easily displace water within the city?
IMO, this city was built in response to the world-ending event from Nika's time.
So:
1. Nika defeats an ancient evil empire, which collapses. This is the 1st World End
2. The world slowly rebuilds, and, by Joy Boy's time, there's a powerful, technologically advanced kingdom made up of multiple races/tribes called the "Ancient Kingdom".
3. Joy Boy fails to keep his promise, and Imu and the WG use the Ancient Weapons to causes the 2nd World End.
4. The WG rebuilds the world in their image, and consolidates power/tries to erase their crimes/the true history.
5. We are here in the current era, with Imu trying to defeat the next incarnation of Nika, Luffy. The Great Cleansing the Gorosei described is another worldwide flood that is looming on the horizon.