Okay so this idea actually comes out of the Syladin ship thread. At some point the idea was floated that we've seen possible interbreeding of spren and human in the Siah Aimians and for some reason something really started nagging at the back of my mind.
Then it hit me. The story of the moon. I think this story is actually about a spren and a human having a child together. I need to pull some quotes together, but discuss for now if you want.
Edit: So here's the passage in question. From chapter 67, "Mishim"
So if true, it would mean that the Natanatan and not the Siah Aimians are part spren. What the implications of this are I do not know, but it could go some very interesting directions.