What flatlines my MMO experience is to see mindless NPC bot creatures that move back and forth a simple pattern, chase you for $range meters, switch to retreat indestructible mode and continue their pattern. Or creatures that seem more complex but after googling you got that standard pattern that works every time reducing it to a test of muscle memory.
I know in the end it is just parameters and patterns but how about adding some chaos into equation? Give creatures something called “nature” which affects their decisions, make their nature affected by weather, number of close by players, time of year, mating season, wind, balance of the zone or maybe even some developer set global chaos parameter that they set when having morning coffee break.
By applying this to NPCs you could make their behavior an indicator of their actions / threat. Players could learn to recognize threat from the actual behavior: beasts more powerful are not scared of you, some want to eat you and beasts weaker will flee. Some are maybe curious, some don’t even notice you, some want to make friends.
This kind of decision tree is hard to pull off but once you get it working it should just be there for whatever creature you will come up with.