Who created the BoM? In Kelsier's epilogue he is revealed to not have powers at the moment so it seems unlikely he made the Bands.
What happened to the BoM between the end of book three and when they were fetched in this book?
What behind the scenes plot went down at the meeting in Elendel when the Bands were retrieved?
How are the medallions created?
These are all questions that I think could be answered in Secret History 2 if Brandon is able to find time to write it as he has said he might.
Also, I can't help wondering if there's more to the architecture of Bilming than just the appearance of the buildings.[Elantris]
The question of what's going on with Harmony is likely meant to be answered in era 3 or beyond.