This question is a little tricky, but I will try to make it clear,

Lets say I am building an online game, not in a mmo scale, but gladly supporting as many players as possible, in a authoritative server approach, and I want really big worlds with lots of AI simulated enemies.

I am aware of a few strategies to save server's CPU by subdividing the space and not processing what doesn't need processing. I 've already split the world by regions, that will require loading times and small transitions, which i think is important to mantain the quality of gameplay when playing locally (alone or even with a couple of friends) because the players won't normally be in more than one or two regions.

But even a region can become pretty big, and have a lot of NPC simulating at a time, how do I handle this without screwing the player's experience? Approaches like one server per region and alike are not in the table.

I am mainly looking for data structures to hold hordes of enemies, and even peaceful NPC. To finalize the question, please note that vehicles exist, therefore its considerably fast to travel within a region, influencing the "when" to cull areas.

Sorry for the confusing question, thanks