They are working mountain caves (dungeons for the mountains, like the earlier biomes all have). And they are also working on the next full biome release. Some of the team have been addressing bugs, some have been reworking animations, wouldn't be surprised if someone was working on balance, etc. There will probably be smaller updates added in with regular maintenance patches along the way.

Looking for mountain caves probably soon. Look for Mistlands probably first half of 2022.