Let's call it a parallel-world what-if.
The story centers on the Three Admirals, Sakazuki, Kuzan, and Borsalino. Sakazuki's personality is probably the most different from canon, while Kuzan should be closer to the original. Borsalino is based heavily on my personal interpretation, and there are tons of headcanons.
Key point: The plot will be written out of order.
I imagined the whole cast as friends with a brotherly bond, but I don't mind if anyone ships them.
The main plot is probably the Marines staging a rebellion (?)
"Don't think only villains can act without restraint. To uphold justice, we can be just as ruthless!", Akainu
"At this point, of course I understand. What I thought was only what I thought. Unless we face reality and our convictions clearly, justice is nothing but empty talk.", Aokiji
"Someone turned sacrifice and responsibility into virtues and set the flames alight. So everyone foolishly rushed to their deaths.", Kizaru
Theme: The military and government in the world of Pirate King are destined to become opposed to each other. To carry out true justice, the Marines will inevitably have to find a way to break away from the government.