Current and past PCs adjusted stat wise to be played as monsters would be fun.
Maps would certainly be great.
Maybe a court of swords time line?
The alignment system,
An index of the tarot system and fortune/bonds created as interpreted into a DnD setting would be super helpful.
I believe there is a thread of people creating a COS tarot set. That would also be very cool to include.
Magic weapons and their lore (who is emperor Fey? Why is he always apologizing?)
Art to reskin some of our favorite monsters, weapons and armor. I find myself wondering what a CoS breast plate would look like, and what other traditionally western weapons and armor would look like in this setting.
You have a pretty significant amount of lore already, certainly more than enough to fill a book. What corrupted the rock gnomes? or the Mogwai? all that fun stuff. And don't even start me on all the Psionic elf lore you have to answer for.