I find it very intresting that we see a pretty and dare say even divide between those that want more monsters, and also want to see an updating of every monster of prior editions and those that want to see more classed monster supplements.
I think I am in the middle ground on this. Monsters were quite static in old D&D, thus you got monsters like the Lizard King and the Flind which were beefed up variations on gnolls and Lizardmen. A Flind is not really needed in 3.5. An Elite Array Gnoll with levels in Monk and a Racial Proficincey in Flind Bar is all it takes. The Lizard King with their tridents were different enough, and tyranical enough that they seem different from Lizard Folk.
A classed monster book would be intresting to me as long as it went the route of Draconomicon and stated a wide range of different CR type creatures. Though I have never used a Dragon from Draconomicon, as a DM I like the fact that if I need a dragon in a jiffy, I know well stated dragons are a book away, ready to be used. A resource like that for humanoids, specificaly high mid and upper level humanoids would be quite valuable, as those are the most time consuming and painful to stat out.
Any such book should have "break points" in the book, stating that such and such a creature can qualify for X PrC. MMIV did that with one of its creatures, indicating that the creature,(which one escapes my memory) automatically qualified for the Assasin PrC.