It's been a while since I thought that after a certain progression in the game, NPC pirates become so weak that it's painful. and I think that can change.

my main idea is to create different power levels for NPC pirate bands, categorized with colors and skins such as brown>red>black>gold. having the base attributes increased according to color, each color would give more doriki and would obviously be more difficult to beat, the idea of the brute pirate captain I also find interesting, pirates with black or gold skins wouldn't attack you before the your 5000 doriki.

villages and bases dominated by pirates NPCS: in the anime pirates can have their own territories and I think that in the game this is also possible, with pirate villages controlling the villagers and having their jolly roger exposed, after killing the pirates in control you become the owner of the region and receives cheaper exchanges with the villagers, (I believe I've already seen this suggestion but I added it again lol.)

wanted poster for NPCS: random event on the map, a wanted poster package drops containing your poster and that of other NPCS pirates that are near you, if you defeat them you gain extra bounty and doriki.

titles in the world: creation of a title progression system, and special NPCs that have it, imagine you create a new world and a newspaper appears showing that there is already a yonkou in that world? you can have titles like super nova, shishibukai and yonkou and better yet, special NPCS can also have these titles and you can find them and defeat them on their ships or in their bases and take their titles and domains, it's a crazy idea like almost all I have but I don't think it's that crazy.

Wanted Poster: Special poses on your wanted poster or NPCS.