Alright. I read this thread last week, but didn't really have anything to add. However, today as I'm listening to The Way of Kings, something popped out at me that is related to magic and the races. In the beginning of Part 3, Adolin goes and talks to the ardents making a wind break, and he notes that the ardents that have used the soulcasters more often start to take on the appearance of the material they soulcast. Could this be a hint to the races? Could the fingernails of the Herdazians indicate that they are descended from people who could soulcast stone?
This is pretty far out there, but it seemed like it might be a clue. I also haven't studied the surges enough to know where soulcasting and surgebinding overlap, but wanted to throw this into the grist mill.