It's hinted at in a few of her chapters in TWoK. She mentions "ten heartbeats" multiple times, as well as "the fruit of her greatest sin." Then in one of the released WoR chapters she talks about a "bright, silvery blade" and lots of corpses. I'll find the quote later, maybe.
EDIT: Bah, ninja'd!
Also: If Shallan is betrothed to Adolin, who is supposed to be gathering Shardplate from the highprinces, would it be too much to speculate that she might end up as a full shardbearer eventually?