I think a great book would be one that details animals and plants together in one book. How they interact together, and not only from a real world perspective. How would animals and plants change/adapt in a magical world? When do animals stop being animals and become magical beasts? All that good stuff. (This book could also include vermin, as well.)
Plus, the book would have more in depth rules for training animals (and maybe even low-order plants and vermin), as well as templates that alter plants and animals in unique ways. A Hybrid template that combines two creatures into one creature (i.e. horse-badger hybrid), an updated Anthropomorphic template that can be applied to not only animals but vermin as well, and new templates that change animals/vermin that work similar to how the Insectile Creature, Monstrous Beast, Multiheaded Creature, and Reptilian Creature templates work (D&D Savage Species). [A template that converts humanoids, monstrous humanoids, and magical beasts into low-order animals would be a nice, twisted idea to include as well (i.e. devolved dwarf-based dogs)]
The book would have special feats and new special attack/quality options that apply to only animals, plants, and vermin, as well as feats and prestige classes designed to enhance PCs so they can better interact with those creature types. Advanced rules for odd druid/ranger companions and sorcerer/wizard familiars (i.e. plant familiars) including optional feats/rules that let multiclass characters combine their animal companion and familiar class abilities into one creature (i.e. monstrous scorpion companion/familiar). Advanced rules for awaken animals, plants, and vermin.
Of course, there needs to be supernatural options such as new spells, magic items, psionics, animal/plant/vermin-based totems, and a few new deities that are dedicated to protecting these creature types.
Opinions? Concerns? Threats?
Cheers!
KF72