Currently in Minecraft we have villages spawn in deserts and fields along with pyramids in deserts, temples in jungles, and witch huts in swamps. I believe that if we add more structures that could be generated in specific biomes they will help add to the atmosphere of these biomes and help newer players find some resources as they start. In Forests and Taiga I think there should be the chance of a hermits cabin spawning. The hermit would be armed with a stone sword and would attack the player if they approached. In the hermits small cabin home there could be a chest with food, leather armor, or book (regular, enchanted, written) inside. The cabin could have a small animal pen next to it containing sheep or pigs.The hermit would look like a villager but with a beard and tattered clothes. In Tundra biomes there should be a chance of an igloo with a villager inside wearing leather armor. The igloo would simply be a small dome like structure with a torch lighting the inside. A chest could be found with fish, gold, books, etc. Mountain biomes include a small mountain fortress or monastery. This would be similar to the temples and pyramids in that it is uninhabited and would contain a trapped treasure room. (Trapped chests coming soon would perfect) The fortress would be a series of close cobblestone buildings and would contain some empty rooms and gardens containing flowers and melons. If you like these ideas and or have suggestions please add them to the comments. Thank you!
Yes that would be annoying if they were everywhere, however the chance of these new structures being generated would be roughly 10-20% if not less so as to maintain many purely uninhabited biomes. The current rate for these structures forming I feel is decent though I do not know the exact chances of them forming. None of these are necessarily very large either and you could easily walk through a biome and possibly not even notice it unless you are really exploring.
So long as the chance of spawning is really low, I'd really like to see all of these in the game as easter eggs that me or my SMP buddies can stumble upon and ransack in our travels. If the spawn rate is high enough that I see these frequently, I'd probably get sick of seeing the same structures over and over again.
I like the idea of every biome having a type of structure, as long as they're rare, like others have said. I'm a little iffy on the idea of most of them being inhabited though. I think they should all be like the desert and jungle temples: empty and abandoned, maybe with some treasure guarded by a trap or two.
And did I mention rare?