I don’t really like the idea of bosses as something that appears all the time or on hordes but if there was one unique creature per biome with its own hidden POI and story, I think it could actually work. At the same time, they wouldn’t need to be something completely absurd or disconnected from reality. They’d work best if they still felt believable within the world of the game more like extreme mutations of existing creatures, failed experiments, or infected animals and humans pushed far beyond natural limits, rather than random fantasy monsters that feel like they belong in a completely different game.
They could also guard unique items or loot that can only be found in those locations, with clues scattered throughout the world pointing to where these places are. Ideally, these POIs would be well hidden, preferably underground, like secret military experiment sites involving humans and animals, designed to survive the conditions of the apocalypse.
For example, if Frostclaw, which I didn’t really like as a roaming enemy, had its own dedicated location in the snow biome an ice cave with its own story it could work much better. Before the fall of the world, it could have been a military experiment. Scientists were trying to create soldiers resistant to extreme cold. Mutagens were tested on prisoners, and human DNA was combined with genes from arctic predators. The experiment spiraled out of control during the evacuation of the facility.
The cave is buried in ice, and inside there is complete darkness. You can see claw marks carved into the walls, and an eerie silence hangs in the air but something is still there, moving just out of sight.
I think this kind of approach would add a lot of atmosphere and charm to the game without breaking its tone, since you wouldn’t see these creatures on a daily basis. They would feel more like rare curiosities hidden in the world. Of course, not everyone would like it, but in my opinion it would be a really interesting and immersive addition.