Onigashima and Enies Lobby same umbrella and Marineford separate but mostly because they get tied at their origins to the main event and conflict, if Magellan played a big role in Marineford I might consider putting them under the same umbrella too. I guess for me Water 7 and Wano set up who the main threat is and it becomes a matter of moving the battlefield someplace else while Impel Down feels more separate from the climax of the arc.