I’ve been doing a reread of the story, and am just now getting back through the Egghead chapters, and I’ve noticed a key piece of dialogue regarding the elimination of Lulusia and the weapon that destroyed it, the “Mother Flame”.
I believe what Vegapunk will reveal to the world within his message is his creation of this weapon, and the government’s usage of it to erase Lulusia. Not only would this completely change the public’s perspective of the government and the Gorosei, but it would parallel Vegapunk’s real life comparison, Albert Einstein, perfectly.
Famously Einstein was responsible for the scientific theories that led to the development of nuclear fission and fusion, leading to the creation of the Atom bomb through the Manhattan project. This was something he felt terrible guilt and responsibility for.
I think Vegapunk feels great remorse for the fact that his greed and lust for knowledge has led to the creation of this cataclysmic weapon of destruction, and that this message will be an apology, along the lines of saying his death will never be enough to make up for bringing this power into the world.
With the Gorosei arriving, we also have the opportunity for the double whammy of revealing their monstrous forms to the public, cementing this idea of the world government’s highest powers being almost demonic entities incapable of empathy, willing to sacrifice countless innocents to maintain their status quo.
This would set into motion more interaction with the Revolutionary Army, and perhaps spark the worldwide rebellion that will lead to the beginning of Imu’s planned “cleansing”.
Theory by charlieminahan (https://www.reddit.com/user/charlieminahan)