In The Hero of Ages, the phrase "shadow of self" was used in referencing the sliver of Preservation's consciousness. Could the next book be talking about the returning slivers of Ruin and Preservation? In that case could the "The Lost Metal" be atium? The schedule for the return of atium to the Pits of Hathsin would approximately line up. I know it's not very solid theory, but the stakes are going to have to be higher the in The Alloy of Law.