I love one-off or short path (9-30 hours of material) adventures, geared to a wide array of campaign settings, so I can fit them into my campaign world easily. I like magic items. I would love to see more clerical whatcha-ma-callums - domains - because the current smattering doesn't always match what I want my gods to represent. I would like more fighter maneuvers. More monsters wouldn't be bad, but I don't really need them. I can generally grab a monster from one of my dozen or more previous edition books and in 5 minutes throw something workable together in 5e.
But adventures, adventure ideas, methods for designing adventures of specific types, whether that be regions of the world, or particular milieus (an arabic-flavored conversion would be lovely), or genres - I don't do horror, but many people love it, and a book on how to make horror adventures, or how to write good free-flowing mysteries, or how to construct situations in which romance/emotion-based adventures work well, would be superb.
As far as book format - I want PDF. I almost don't buy physical gaming books any more. I use my tablet for reading, and I either copy-paste to text so I can edit as needed, or I just print, cut out the parts I want physically, and paste back together and scan. I know, I'm crude, but it ends up getting me what I want!
I rarely notice authors' names (sorry!) and almost never the illustrator. Those things just don't really matter to me. I buy from Frog God/Necromancer because I've always liked your stuff. I trust you to pick good writers, but who they are doesn't really impinge on my notice.
If I DO buy print books, the thing I want more than anything else is easy to read print. I'm almost legally blind, and I find 10 point font indecipherably small. Give me good bold black print on white paper. I've had to add page tabs to all my 5e books, because I can't read the page numbers at all. Give me minimal fanciful art that I never look at anyway. Library binding is great - I'm a librarian and I know its worth, but I still want non-glossy pages for easy reading, so the ink doesn't smudge, etc... I have to send my glossy graphic novels back to the bindery often enough to know that no matter the quality of the binding, that glossy paper WILL slither out. It's cursed, or possessed, or something.
Maps, however, are VERY important to me. I want good maps. Logical, realistic, and fun. Lots of them. Player maps as well as DM maps. I'd have KILLED for a player map of the Wave Echo Mines. I'd have killed for a map that made a REASONABLE functioning mine, too, so I didn't have to entirely redraw it. A mine that didn't have an easy way to get the ore to the smelters? Ha! come on! THINK logically!!! I hate it when I have to spend 15 minutes figuring out just which way the stairs on a given level are running. Half the time I never do.