The world needs to have purpose. Things need to have purpose, even mobs.
A good example I once heard of was, imagine walking down a path, say you needed to find a town to sell some loot you just recently plundered in a nearby cave. But the path becomes blocked when a band of orcs rush on by towards the town. They slaughter everyone and all npc’s in it Occupying it until further action is taken.
MMO’s need emergent content, things that happen by no consequence of anyone single person. I’d love to see a game where there’s purpose to everything in the game. Why did that orc raid invade the town, what happens afterwards, do players try to liberate it, or do they brush it off and does that raided town turn into a bustling orc encampment that threatens future settlements?
I really like roaming mobs. I think AI with pathing is far more interesting, it can allow for all kinds of gameplay.