I started a fresh new Superflat world with the "Overworld" preset. I changed absolutely nothing with the seed and started playing. One of the first things I found was a Desert Temple, and not too far from that was a Witch Hut. After doing some exploring, I found some more of these, plus a Jungle Temple and a couple Igloos. The Villages that normally spawn in the plains still spawned. I haven't found any other out-of-place structures and haven't tried this in any other world type.