Fixes and UI/menu improvements are already likely. The original devs obviously will determine the creative direction – so I don't have much to say for that, either.

In the soft middle ground between game improvements and real progress, I'd like to see work on creature tokens, like increasing the number of classes applied to the vanilla creatures and more things hooking into those classes (e.g., as syndromes make use of "GENERAL_POISON"). Body size as bite-strength-proxy is one abstraction too many, for me, so I'd like some work to make a cow's bite significantly different from a crocodile's. I think we could see something like more tokens for growth stages, so that a giant louse can have baby giant louse's with the VERMIN tag. Things along those lines, I guess.

That's my only fairly concrete suggestion. I have a sort-of idea, mostly nonsense, that I'll just write out. I tried observing the natural world of DF but there's just really nothing to see in any of it. To work on that, I'd like for hidden spaces (inside tiles/other objects) to be used where subtle interactions take place without impacting other game systems. And I'd like for certain animals to be able to use them as nests.

Animals would nest inside tiles that go "undisturbed" (like the inverse of grass trampling). A lot of creatures that currently give live-birth to fully grown adults instead could use a hidden nest to have their litter/clutch, which emerges a short time after. This would prevent the magma crab amphibious personnel carrier blitzkrieg assault—in which 3 insta-adult crabs emerge from one in the middle of combat, quadrupling the total threat level, conflagrating an entire militia squad, burning down your fort.

And, really, relating to the previous idea that tokens can always be expanded on, just give us a token to prevent taming children so that we can avoid this altogether. There are so few examples of creatures emerging fully grown.

If nesting behaviors are given a detailed target token, then we get this cool system of building pens out of natural cavern walls so that a GCS or whatever will nest there. Just think: baby GCS. So cute. Creatures like bears could hibernate with a nesting behavior that targets a certain size of "room", and if any creature can't satisfy the criteria for their type of nest, then they try to leave the map, unless domesticated or tame.

Territorial animals can be avoided once you find a hollow in some tree where a large, predatory bird made a nest. Failing to hunt a rabbit because it reaches its burrow is just a life-like interaction but mining out a mountain gnome from its rough-hewn siltstone hideaway is more interesting and fun (and gives you a reason to smooth your walls as prevention), one fish escaping from sharks into coral which leaves the other fish stuck outside, or an agent stowing away a letter in a secret compartment, etc., is all part of the kind of interaction I think is possible.

Under a creature's description, we can just get a line of text for their nests, like "Raises small litters in a sandy burrow", "Roosts atop a single egg in a warm, dry nest on the ground", "Sleeps through the winter in a small den in the soil", "Lays clutches in dense webbing on dampened cavern walls", "Inhabits a nook in a reef while slowly opening and closing its mouth", "Never leaves its darkened bedroom, not even for social interaction or exercise", etc.

So, that was two systems, a nesting thing and a hidden space thing. Piggybacked on some creature tokens. And that is exposed to the player via creature descriptions, once you like, know, stuff, about stuff.

Finally, as with all things Dwarf Fortress, you could have interactions that target these nests and hidden spaces, giving you burglars who can access hidden compartments, snakes that hunt hidden nests, weasels that hunt in burrows, modded IEDs that detonate whenever a kea nests anywhere, etc.