I have a theory.
I believe that the 2018 Tolkien Calendar will feature Alan Lee's artwork. More than that, I believe it will be from the forthcoming Beren and Luthien, making it a "2018 Beren and Luthien Calendar."
How did I arrive at this conclusion?
Every so often, I check out sites like Amazon UK or Book Depository for listings of upcoming books (not limited to Tolkien). I saw that the 2018 Calendar was listed, so I went to the page there. It stated that Alan Lee is the illustrator. Also, it says this:
20 b/w, 12 col illus.
Checking out the page for Beren and Luthien (also illustrated by Alan Lee) it states this:
20 b/w illus, 8 col plates, Index.
Based on this, I believe it's fair to guess at this point that I may be accurate. Please note that when The Children of Hurin was published in 2007, the 2008 calendar featured artwork (from both the book) by Alan Lee.