It's a cool idea he got, kind of a bonus for following his works closely.

When I attended that signing, he didn't reveal the setting was developped for a specific series he had worked on early, though it doesn't surprise me it's the case. Sanderson has a bigger "back burner" than any writer I've heard of (though it's perhaps just because he doesn't mind talking about his developping projects to his fans - in fact seems to love to, unlike most writers).

It's all fairly vague in my memory at this point, but I remember he spoke of souls, levels of existence and stuff like that, and how the cosmologic/spiritual aspects in his books all follow the same (as yet unrevealed) rules, and he mentionned Hoid (not by name at first, but when someone brought the name up he started using it.). He threw bits at the fans, but it was rather obvious he didn't want to give much away. I remember he put an end to the discussion at some point by announcing he'd rather show us the concept art he had produced to help him visualize the Way of Kings setting and characters (pretty cool, his friend who did it for him had a very manga-like style), then he showed us art for a series featuring chalk magic, though IRRC his plans for that series as he described them back then have changed completely (at the time, in part because of WOT, but mostly for the fact Doherty had just bought Stormlight, he was planning to sell the setting/storyline to a friend who would produce it as graphic novels or comics, and he would act strictly as story consultant, but I vaguely remember seeing later that this got back to a novel project by Sanderson himself. Not sure if it's still YA as the GN project was supposed to be.).