Think that'd be hard to implement, cause what would happen if the player kills the NPC. Would an admin need to check the areas NPCs are at and respawn NPCs if killed?
And what would happen if the NPCs chase a player too far.
Something like that would likely need:
-A counter of how many of a particular type of NPCs exist in the world.
-Areas the NPCs are constricted to (if needs to be set) and some form of AI to handle NPCs whose target leaves this area (whether they run back to where they were or if they just despawn).
-Check if the chunks (areas) in which NPCs are constricted to are loaded in order to spawn. (As not to try to spawn entities on unloaded chunks)
-Respawn time (Ie: 5 minutes), not to have admins have to constantly respawn the NPCs in those areas should something happen to them.
-Possibly set the drops NPCs have. There'd be no point of a tough NPC which drops nothing.
Don't get me wrong, this would be pretty dang awesome to implement! Think it's a bit early for this plugin to start considering such a tough task. Like I've said before, there's a lot of potential for this plugin and likely for other plugins to appear that require this plugin and can handle more complex tasks outside the scope of NPCs.