Things I would really like to see in this book of fiends:
more instances of chaotic evil manifesting itself in the representations of demons. (Hordelings, for instance, would be fantastic!). Demons that change shape uncontrollably each round. More varied demonic creatures: demonic plants, demonic constructs, demonic undead, demonic beasts. I don't want all demons to have the same basic attributes of Fire Resistance 20,.... etc.
I grow weary of the old evil-vs-good take on everything. I want more law-vs-chaos approach. I want the chaos part of their alignment played up. The heaving chaotic masses of ultimate vile Evil.
As a player, I never bought into the Planescape thing, TSR were producing so much stuff, and my dollars were limited in those days. And I was put of by all the berking. So I don't really care for backwards consistency. Though I understand it may be important for others, apparently.
I want cool ideas I can use. Places, names, feats, spells, powers, mythologies, histories. Brief descriptions of various abyssal planes, and their denizens. (not detailed stats, just the bare minimum to get fresh ideas).