I voted #1. I could see the space a "Classed Monster" book would follow, but the problem I have with it is that doing so is a little too limiting, in one way or another. Let's start with what the purpose of a "Classed Monster" book is - it's to save me (the GM) time. No more, no less. I don't spend my valuable prep time cranking out an advanced critter, I just look up the Monster/Class/Level combo I want, and there it is.
Examples:
1) Full monster NPC descriptions - stats, personality, motivations, plot hooks, etc. The more specific/detailed you get, the more adaptation I have to do to fit things into my campaign. At the worst end, I might as well just be rolling my own. Plus, the format they're using (by necessity) is very long, which dramatically decreases the number of different Monster/Class/Level combos they can detail, leaving great big holes in the book. At best, you could get "Tribe" books, focusing on a single kind/group of monsters, but even then, it'd still have holes and adaptation problems.
2) Old 1e "Rogues Gallery" style - just the stat's, Man. Preferably in a highly condensed format that can be presented in one monster/class/level per line in a table. Think the back of the old 1e DMG, or the NPC section in the 3e DMG, but applied to everything, instead of just human/PC class combos. I'd actually prefer this, but given WotC's current stat-block track record, I hate to think what would come out. It'd probably kill John Cooper if he reviewed it. Plus let's face it, it'd be *boring*. *Incredibly* useful, but *boring*.
3) A 3.5 edition of Savage Species that collects (and corrects) all the rules for advancing, templating, classing, and creating critters. And provides "turn the crank" instructions. But now I have to do all the work myself, which is what it's supposed to do for me. (Still it'd be nice to have complete rules that work and are accurate.)