Fiends are the primary antagonists in my campaign—demons in particular. As has been pointed out, fiends have been dealt with extensively in the past, and they've been done well. I have the 4e Demonomicon and both of the 3.5 Fiendish Codexes, both purchased from DriveThruRPG (which means I only have a physical copy of the former, which is sad). All three are terrific pieces of fluff, but I didn't play either of those editions. I played AD&D for two decades and then took two decades off before coming back for 5e. I can infer a bit about 3.5 and 4e rules from splatbooks, but without the core books, a knowledge of the rules, and experience playing with those rules, the texts are ultimately foreign to me. I might as well be looking at Pathfinder books. I want a tome on fiends that uses rules I'm familiar with—a book for the game that I play.
That said, I'd like to see a fiendish tome with three sections. In addition to demons and devils, the abyss and the hells, Wizards should take a stab (and hit the mark) at an exhaustive section on Hades and its daemons. The Gray Waste is wide open canvas on which to paint yugaloth culture and introduce some new NPCs who could stir up the Blood War. They even could present it as a collection of fiendish dossiers assembled by Shemeshka.
And crafting a huge chunk of new lore might keep them from retconning perfectly good extant lore. I'm still not over Graz'zt being a devil. As if.