These are most certainly already planned features, but with the upcoming reintroduction to civilization sites, I think these sorts of related topics are worth a discussion.
Right now, subterranean animal people are rather uninteresting. Their sites consist of a few spearman, blowguners and some tanned leather scattered around. They will only attack when encountered, and their wood weaponry really can't compete with a competent axedwarf in iron. Not nearly as dangerous or threatening as the old snakemen from the cave river.
Adding some variety to the various creatures would be a good first step. Not all animal people should utilize spears or blowguns and organize in the same way. Ratmen, for example, are too small to pose a threat of any real kind in small numbers. Different animal people could have different tactics to fully utilize their potential. I could see ratmen using vast numbers to their advantage to raid your food stockpile, but lacking the more advanced tech of other animal people, like blowguns. Amphibian and cavefish men could dwell in cavern reservoirs, taking potshots of your livestock or passing dwarves. Antman, if given enough time, could tunnel through your defenses and raid your fort for food or valuable trinkets.
Sites could also see some expansions. Since this is a camp where these creatures live on a day to day basis, having children and crude craftsman wandering around the site would add some realism. Maybe some could construct crude wood huts similar to those in human sites. Again, not all creatures would have similar infrastructure. An antman site would vary drastically from an olm man site.
Wooden weaponry isn’t really an effective counter at any stage in the game. Blowguns do have poisons, but they are not always lethal or effective. Like elves, some other advantages are needed in order to balance their combat potential. Domesticating and mounting large predatory creatures would be a good start. Forgotten beasts could also become objects of worship for a particular camp and some may just be smart enough to exploit this position. A raid of batman would be a lot more threatening if they rode on giant bugbats and were commandeered by a quadruped with deadly dust.
But the issue still stands with behavior. Subterranean animal people won't really leave their site or interact with your fort unless provoked. Since these are hunting/gathering creatures, some animal men could send out small expeditions to explore the surrounding cavern, both on and off your map site. Once they find your fort, they could begin staging more complex assaults, similar to goblin ambushes. These ambushes could later expand into full-on underground sieges, kind of like a full occupation of your cavern with hostile animal people, tamed giant critters and forgotten beasts.
The potential for friendly relations is also pretty vast, but I feel more complex civ interactions in general would be needed first.