Sabo is Luffy's brother and Oda planned his existence from the beginning. Oda made it very clear that Sabo would not become Luffy's subordinate when deciding between Luffy, Ace, and Sabo who they would be - all were to become captains of the pirate crew. However, Sabo, unlike Luffy, became the first lieutenant of the Revolutionary Dragon Army. It can now be said that Sabo is inferior to Luffy, if not in strength. then in rank.
After the events in Wano Country, Luffy became one of the four Yonko, and even though Sabo is called the Fire Emperor, that doesn't make Sabo a match for Luffy. However, Sabo is Luffy's brother, and Oda can't let Sabo be so inferior to his little brother.
That's why I'm convinced that when Luffy becomes the Pirate King, Sabo will become the Revolutionary Army Chief, and then the brothers will start the last war against the World Government together. This way, Sabo will not be inferior to Luffy as the head of the Revolutionary Army, and they will be equal allies.
What does this mean for the plot in the future? Dragon, Luffy's father, will be killed before Luffy becomes the Pirate King. Perhaps the death of the Dragon will occur in the Egghead island arc. Why?
Think about who Vegapunk is and why he began to study the ancient history of Ohara? Why was Vegapunk the one who turned Kuma, a member of the revolutionary army, into a living machine? Perhaps there is some Dragon's plan, when Kuma, already turned into a weapon, will be able to sneak into Marijoa without arousing suspicion, and carry out some kind of diversion, which will allow the Revolutionary Army to start an assault on Marijoa.
Therefore, without a doubt, Dragon cannot allow Vegapunk to be killed by the Marines. The battle for Egghead Island will be much larger than Marineford: 100 ships, along with the Admiral and Gorosey, are heading there, while in Marineford there were only 50 ships at the Watch. The appearance of Akainu himself is not excluded, in case if Dragon arrives on the Island. And although I am sure that Vegapunk will be saved, Dragon himself will die there.
But how can you believe that Oda can kill Dragon in this arc? Think Whitebeard was killed in one war arc even though he was introduced much earlier as the World's Strongest Man. Dragon will share the fate of Whitebeard, and the news that will shock the whole world will be the destruction of Dragon the Head of Revolutionary Army.
