The tundra/arctic biome is barren wasteland of snow and ice that doesn't serve much of a purpose other than to get snow for snow golems. I think this is a cool biome that is completely overlooked due to the lack of content. The only mobs that specifically spawn there are stray and polar bears. Tundras irl have, like many biomes, a diverse ecosystem with many different plants and animals. I prepose that more gets added such as snowy owls, elk, penguins etc. I also think that foxes should get a coat change to white when they enter it. I also think there should be added structures like an ice temple or an abandoned adventurer's hut.
Igloos are a completely unique spawn in this biome, as well as a unique type of village, and foxes already have a white coat when spawning in snowy taigas, which can be next to the snowy tundras. I think an ice temple would certainly be cool though, maybe in the ice spikes biome. And while I don't think elks or penguins would necessarily add that much I'm sure owls could use a unique mechanic! Although they definitely wouldn't spawn in the tundra, since there aren't any trees.
Maybe there could be a new type of tundra, instead of "snowy tundra" just "tundra", with reddish grass, sparsely placed spruce trees, and more passive mob spawns.