Do you think illustrations in books are going to become more common in the future? Sanderson seems to be enjoying exploring the idea of using them as part of his writing process and as another way for the reader to interact with the work (particularly with Elantris and Rithmatist).
Gosh, I sure hope so. I'm a huge fan of illustration, and I think when it's done well that it really adds value.
I just picked up the George Martin collection of his Hedge Knight shorts, A Knight of the Seven Kingdoms, which has 160 beautiful pen illustrations by Gary Gianni. And I love the huge, hardbound Kaluta-illustrated of A Princess of Mars that came out last year.
Beyond that, I'm a huge fan of what Brandon's been doing with specifically illustrating in-world documents. We don't really do "scene illustrations" in Stormlight, and we only do spot illustrations in terms of the chapter icons. Some of those are more contextually descriptive, like Lift's pile of food (which is doing double-duty by being both thematic of her power and hinting a little at Rosharan food typs. Lots of gourds, shelled meats, berries and seed-breads) or the straightforward picture of Adolin's Plate, but for the most part we're pretty careful about making every illustration also serve as an artifact.
A page of Shallan's represents a page in her collection. A map of a location is a map on someone's table. An image of Alethi fashions is part of a regular delivery of correspondence that Adolin recieves because that's how damn rich he is.
Not every book would benefit from that sort of supporting content, but some might. I'm reading Jim Butcher's magical airship adventure The Aeronaut's Windlass and enjoying it, but I could really go for a series of illustrations on just how these ships are meant to look. He gives a pretty good description of one of the main vessels... 190-some pages into the novel, after a couple action scenes involving that ship, and I'm still kinda unsure of what the other ships look like.
Lastly, given that there's plenty of precedent for books that are well-suited to be adapted for other media, it's not a bad idea to establish aspects of the property as the author intends while he's still got some input. Once it gets licensed, that opportunity is often reduced or lost.