I agree with your idea in general. I like the roaming packs/flocks and the patterned behaviors. It would also be neat of a roaming predator picked off a marmot/rodent every now and then as they wander. I would still keep their zone to return to, just have that be what drives the roaming aspect. It could be as simple as moving the zone 10-20 feet every 5 minutes along the predefined path, then the mobs follow it. Stand around for 5 minutes, move a little, repeat. Oh, it's raining? Move to the nearest tree until it passes. It would certainly add life to the overworld.