Like other people has said, BFA is not a pirate expansion. It is an expansion about an Azeroth stabbed by Sargeras' sword, a wounded world, harvested by the Horde and Alliance gone to war. Azshara and N'zoth rising as the ultimate threat of the expansion.
Now, if you want a Pirate expansion -- the way I would see it is: The South Seas expansion including Plunder Isle. If Lady Ashvane escaped like Gul'dan did at the end of Warlords of Draenor... Maybe it could be Ashvane rising as the main boss uniting all Pirate clans together. Each clan boss serving Ashvane, and becoming like the Lords of War kind of thing. We defeat each boss in their proper zone until we reach the last boss raid, end-game.
However, a full pirate expansion seems a bit boring. There should be more than just that. Only benefit is that pirates have members of basically all races in Azeroth: Hozen, Tauren, Undead, trolls, goblins, gnomes, Quillboar and even Saurok (as seen in Daelin's Gate).