The cleanest way for Oda to start the final war is to have the Gorosei be killed off in Egghead. When this happens it will leave a hole in the chain of command between IMU and the WG, and create an opportunity for multiple factions to claim leadership.
Garling and the HK seem to be the next highest CD besides the Gorosei, so they will have a claim, while Shanks as Garlings son or nephew will also have a claim, if he can take out Garling.
Blackbeard Pirates have also created a claim potential for themselves if they try to say they have the last remaining Gorosei on their side, with Katarina transforming into Saturn.
The Marines faction will also have a claim due to Kong being Commander and Chief, and Akainu having command of the main military arm of the WG, the navy.
Most likely Straw Hats will also find out that the D-clan were rulers of the previous ancient Kingdom giving Luffy a claim as well.
Garling and the HK seem to be the next highest CD besides the Gorosei, so they will have a claim, while Shanks as Garlings son or nephew will also have a claim, if he can take out Garling.
Blackbeard Pirates have also created a claim potential for themselves if they try to say they have the last remaining Gorosei on their side, with Katarina transforming into Saturn.
The Marines faction will also have a claim due to Kong being Commander and Chief, and Akainu having command of the main military arm of the WG, the navy.
Most likely Straw Hats will also find out that the D-clan were rulers of the previous ancient Kingdom giving Luffy a claim as well.