Love your art style. Reminds me of adventure Time. My thoughts were going to animals aswell. Deer, bunnies, birds, fish. Sheep for the grassland. Make them eat aswell. Makes them more alive.

Awesome art style! Is the river animated? If not you could add move my water animations to the river. Snow falling on frozen peaks as well. Also on the other biomes perhaps some shadows of clouds that would move around??

Looking a the ice biome specifically, in would take insipiration from places like alaska. Its too cold for sheep, but things like moose? Bison? Large animals that can deal with the cold well? Those are your go to.

You could have ice giant shephards, or snow owlbears. Is this near a glacier? If so, glacier silt acts as quicksand and could be an interesting hazard that fits with the theme.

Birds flying by

Honestly I feel it's okay if the ice biome doesn't feel "alive". It's usually the unnatural stillness and quiet that people find either very disturbing or which they appreciate about it. Has a sort of aetherialness to it.