When the Egghead Arc was starting, no one was sure what the final battle would look like or whether there would be. The main antagonist could have been anyone from Dr. Vegapunk to Rob Lucci to Admiral Kizaru; the answer has remained nebulous throughout the arc. To make matters worse, Luffy's new Sun God powers make it difficult to imagine anyone giving him a hard time at any point in the immediate future. Thus, the climax of the Egghead Arc and the world-altering incident set to happen in one day have remained a mystery.
Source：https://www.cbr.com/one-piece-theory-luffy-vs-saturn/
