A while back I removed AI system I had been developing for some time and merged it to a command structure. Maybe I was looking for a simpler solution, but after some thinking it's obvious that there must be a separate AI logic with the original primary/secondary task idea.
How tasks will be organized is not yet clear, but each of the creature has to keep something in mind, a goal to which it is reaching. A number of things can change the goal, but not secondary tasks which involve things like swimming or some other circumstance as a temporary block between the creature and its target or goal.
I'm too dumb to use any sophisticated AI algorithms, but I think you don't need any of that to make creatures do what you want. Also, algorithms tend to produce predictable patterns that can be exploited. I intend to create an unpredictable AI that can sometimes make huge mistakes or forget something important. If I can make it so that the player will see those mistakes then it's going to be cool.