My vote? Elementals.
Elementals - For being fairly interesting and largely unrepresented in 3e. It could update the Paraelementals to 3.5, update the Quasielementals, include deity writeups on the Archomentals (good and evil), include info on Djinns and Elemental-like creatures (Efreet, Dao, Marid). Hell, it could even feature the Quasi and Paraelemental planes as 'Planar Variants.'
Giants - Pretty big fantasy staple creature here with a lot of old 'classics' still not updated (Ogrillons, I'm looking at you!). Lotta room to cover here.
Fey - Not my favorite creatures, but they could use some support.
Aberrations 2 - The Return! - Another aberrations book focusing on the Psurlons, Ethergaunts, Cloakers, Avolakia, Naga, and Destrachan. Won't happen though...
Outsiders - I think a few books focusing on the good, evil, and neutral creatures would be good, but, as has been said, they've already gotten a lot of support elsewhere. Besides, if a fiend book is ever done, it won't make me happy unless it includes ALL the Yugoloths PROPERLY updated to 3.5, the Molydeus, the Nabassu, and an Epic conversion of the Baernoloth. Sadly, I don't see much of that happening...
Plants could be interesting, but they're as disparate as Aberrations are. Same with Magical Beasts... though that could be an interesting book too!
Constructs, Animals, and Oozes would be my last choices for this style book. Mostly because not much can be said about their culturals, societies or lifestyles (Gelatinous Cubes enjoy slithering down 10 foot wide Corridors, picking up trash, and eating unwary adventurers... ummmm, next Ooze?). These creatures could be handled well with some short, 96 page softcover books. Anything bigger would be a waste of space IMO.