if using the Star Wars sets as guides, it's worth mentioning the new "ambush on mandalore" set, which is basically two half battlepacks in one box. with two imperial commandos, two mandalorians, and a small terrain build with minor play features.
https://brickset.com/article/109680/review-75373-ambush-on-mandalore-battle-pack
i feel like this would be a popular sort of set for Pirates, perhaps with with two Imperial Guard troopers, and two pirates, with say a stand of palm trees on a sand dune and a small guard post. maybe a rowboat.
you could then build off this with a larger set with some more terrain and more elaborate minifigs on both sides (analogous to the "paz visla and moff gideon battle" set which the smaller set can link to directly to to create a larger scene/play scenario) say, a small brick built boat (bigger than a rowboat, but still limited parts count) with two pirates of a somewhat fancier style, vs an imperial guard cannon crew.
then you could do stuff like an imperial guard troop pack and a pirate pack, each adding both more basic minifigs and accessories, and brick built terrain bits for play use. which would suppliment whatever ships and islands form the bigger sets in the line.