Some mechanics (most of them) require an update of NPCs in crews, something we already know is coming, so here we go
Diver: NPCs from your crew or a friend can dive to rescue Devil Fruit users who are at the bottom of the sea
Celestial Dragons and their mounts: Celestial Dragons are mounted on NPCs of Brute Pirates, when you kill the Celestial Dragon the Brute Pirate will join your crew.
Cyborgs can improve their own bodies: this could be in a separate topic, but here we go, I think cyborgs could manufacture new parts for their bodies, like a bionic diamond arm or something like that.
Cyborgs make their own pacifists: Advanced cyborgs can manufacture pacifists like iron golems, and control them like dogs.
Pirate NPCs invite you to join the crew: a pirate messenger will deliver a letter from a notorious pirate inviting you to join the crew, joining a notorious crew will cause its captain to give you gifts over time but in exchange you will earn Bounty and less belly.
Bounty hunters BH NPCs are coming, so this may come in the distant future: whiskey peak a village full of BH, entering this village may cost you your life.
secret base of revolutionaries: the name says it all...