What if you could just find stacked pillers of blocks that players would normaly stack with such as dirt, cobblestone, and oak plants? these block stacks could range from 4 blocks tall, to maybe even world height.
Nothing crazy, just atmosphere.
#SUGGESTION
regal frigate
stray sentinel
Simple, would need to be off by default but would be kinda eerie knowing something player built that you didnt build resides in your world single player world..
Here's an updoot.. ^u^