This is my theory, inspired by this Reddit.
What do you think about it?
The purpose of the World Government is to prevent the use of Ancient Weapons. This is because they can control the “Army of Giants”, huge and powerful giants (Humanoids and not), with the ability to destroy the (emerged surface of the) planet, literally.
Before and during the Void Century this Army was used negatively (E.g. the war between the Great Kingdom and the future World Government).
Originally, there were Imu and Nika (A Giant), the Moon and the Sun. To stop the use of the Giants, the two they decided to trap (and maybe control by letting out on will) the Army of Giants, creating the Florian Triangle by putting the Three Great Gates in the sea.
The creation of these structure, however, took the life of Nika, whose body/straw hat is now kept in Mary Geoise’s Chamber of Hibernation. On the contrary, however, Imu probably thanks to this event has managed to have eternal life.
The prophecy of the “will of the D.” (D of Dawn) describes when Imu and Nika will finally get back together, opening the Big Three Gates. The day of the meeting will be during an Eclipse (Moon and Sun), followed by a Dawn. This date can be predicted thanks to astronomy, so it is easily deduced by a scholar, or by Roger (who knew he had arrived too soon at One Piece).
The bearer of Nika’s will to free the Giants is called “Joy Boy”. The will of the Giant was always to bring happiness (Joy) and freedom to the world by using the Army of Giants to destroy the Red Line and create a single great sea (One Piece) where all organisms can live and feast together. This will, however, is opposed by Imu, who instead (Living since the Void Century) knows what the Weapon of the Giants can do and wants to suppress its liberation at all costs, preventing anyone from knowing about it (Nika HAD to trap the Army to prevent others from using it negatively, but he wanted to do his will).
The real enemy will be Blackbeard, not Imu or the World Government. Luffy (Joy Boy is the “heiress of Nika’s will to bring happiness and freedom to the world with the Army of Giants”) will meet Imu. The gates will open, and finally the Army will be free. To command it there will be the three Ancient Weapons (Poseidon - Shirahoshi / Pluto - Momonosuke / Uranus - ?).
So, when the Army is free, there will be two ways to use it: destroy the Red Line to unite the world or control the world by force. Luffy will want to achieve the first goal. Blackbeard will want to reach the second.