I am thoroughly aware that this topic has been beaten to death, but if we all agree that animals should have more complex behaviors, it is more likely for the devs to see it and make a move on it. Which isn't to say we should pressure the devs.
(Warning, I am autistic, I speak densely and bluntly. Walls of text ahead.)
That being said, I'd like to see more complex animal behaviors, including; mating, rearing, hunting/eating/drinking, pests, and curiosity.
First off, mating behaviors of animals are very complex and hard to boil down to "This animals behaves this way and that animal behaves that way." In particular, I'd like to see animals find companions of their own and join them. We see this in wolves, pigs, goats, and deer in game, but I'd like to see that expanded maybe a little more. Wolves usually travel in packs of five to twenty, and although fending off a pack of twenty wolves would be impossible in a single player and possibly multiplayer world, I'd like to see larger packs, make them a little more of a threat. Same goes with bears, they are *mostly* solitary creatures, expect when they fish and mate, in which tons of bears gather right after spring time. It would be nice to see more bears together in larger herds. This may impact performance, so I'm not asking for the game to become unplayable for the sake of realism, but I would like to see animals spending time together as they would in the wild.
Rearing kinda follows this topic, in that instead of seeing one or two baby animals following the parents, maybe we could have dens/homes for the animals to defend. Wolves roaming the wild with their pups isn't super common, so maybe some caves could have less "spelunking" feels and more dark dense opening in the ground in which animals could hide. In the same sense that certain animals will use areas are a sort of home base, maybe certain areas will spawn types of animals more often. For example; plains will spawn deer, goats, rabbits, and... other animals that live in plains, and they are likely to return there to graze as a sort of home. While caves will likely spawn larger packs of wolves or bears. I'd also like to see predators in particular defend their young.
Hunting, eating, and drinking, in my opinion, need the biggest overhaul. I get really annoyed that not only do we not see animals other than bears hunt, but predators hunt the player character on sight. In reality, most animals avoid humans, even if they don't have a lot of experience with them, because of our size and complexity. Animals like wolves only hunt humans if absolutely starving, and are more likely to defend themselves or their young from humans and don't always attack to kill. Which, to be fair, wolves in game sort of behave this way. But what I'd like to see more of is complex hunting strategies. Wolves are ambush predators, instead of running up on them, maybe they could sense us from a distance and hide in the underbrush, leaping out to attack when we turn our back. Bears usually only defend their young against humans, black bears are very timid and stay away from humans rather than attack on sight, brown bears are more aggressive, but even they can be pretty nonchalant around us because they know they could kill us. In the same sense that they hunt with complex behaviors when they do, I'd also like to see animals gather around water sources. Even though drinking isn't currently a mechanic, it would be nice for animals to still behave in realistic ways.
Pest behavior is a big one for me. As much as a I hate pests in real life, they fill an important ecological niche, and can add a realism to gardening in game. Animals like racoons, gophers, rabbits, squirrels, and much much more will root around in your garden and uproot plants, eat your plants, and all round cause havoc. This kind of happens if you have a garden without walls in game, goats and such will at your plants, but I'd like to see animals that can scale fences, or dig under walls to get into your garden. they don't always destroy all of your garden, in fact I have a garden right now that has been terrorized by the local animals, but they only hurt less than half of my garden. Either way, it would be nice to have some sort of way to keep gardens from becoming over blown fields of produce in late game that are both impossibly giving, and very easy to maintain. And this doesn't just apply to gardens, all animals have pest behaviors. For example, wolves and bears WILL try to break into homes or boxes with food in them. Although it make predators op, maybe the need for food and the next mechanic can be combined to make wolves and bears occasionally try to break into homes. Could make home defense more interesting.
Which leads into the last topic, curiosity. All animals have levels of curiosity, some much less than others. Foxes steal food from houses and people all of the times, gopher sneak into gardens and eat your food, squirrels follow people around and will befriend humans who give them food. I'm not saying these are all mechanics that are doable in game, but a level of curious behavior and theft could easily be doable by in game animals. For example, if a chest inside a house with an open door, or if the chest is outside, contains food, foxes will steal that food. They could also snatch food out of players hands if it is the item they are holding. Wolves and bears might inspect a door rather than charge at it blindly, and if the reward is big enough, might decide to break down the door in order to get the food or animals inside. This would be particularly realistic in the sense of barns, because predators break into barns all of the time rather than hunting down dangerous humans.
These are all pretty in depth and decidedly complex, so please feel free to add, subtract, or diversify how they could all be accomplished. Some of these might just make the game more cpu heavy and therefore aren't super doable, and that's understandable. I would like to see some more complex behaviors because it kinda bothers me that wolves and bears attack on sight, every single time, and that they don't truly occupy ecological systems. Idk.
TLDR; the autistic woman want's animals to behave like an ecological system.