|Jameow Goblin Squad Member|
I think it's the sort of system you want to have in by release, as it has repercussions for resources, and is a system that isn't totally different to many of the planned features, in many ways it's just about writing the conditions for different creatures properly and not making it too static or too random.
Everquest next is planning a system for spawns (given the dynamic world they are creating) where creatures have likes and dislikes. The example they used is orcs: they like gold, they like attacking roads that are not heavily guarded, but have enough traffic to make banditry profitable.
So orcs like roads with minimal soldier patrols. Change the guard patrols, attack the Orc camps too much, and they'll move elsewhere.
It's a similar concept, you could do it by setting a bunch of likes and dislikes for various creatures to determine their spawning patterns.