My theory:
Taldain maybe? The "Waves" could be sand? The colour scheme doesn't fit though. It could also be a new world, as Brandon did mention that.
Random non-Cosmere book.
I saw some theories about the two characters as Siri and Susebron. I also saw one about Nazh and Khriss. I like these ideas, and I agree with them.
There is a line in one of the letter from Frost to Hoid
On the cover of book 4 is a gemstone like thing. It could be a fabrial... Or we could have a Hoid POV book!