Would someone like to share an idea?

[ChaosDefender24]

Really, the main objection that druids and the like have about things that go on in the world are the undead. Undead are a perversion of nature and anyone who gives life a healthy respect tends to attack the undead on sight. Any real infestation of undead that's encroaching on a natural area should be enough to get the PC's riled up. I won't provide any more specific ideas on the plot; this is so general and so easy to work with that you can probably run riot with this one without help. Although I would suggest that you get as bizarre and weird with the undead as possible; troops of skeletons marching through the woods are one thing, the players finding flayed animals (be careful with something like this if your players are squeamish) and then shortly being attacked by particularly furry skin kites (Libris Mortis) is another.

Or you could have some evil force (I like evil outsiders or something similarly perverse, perhaps an undead for double trouble) actively working to corrupt nature, poisoning water, grafting stuff onto animals, and generally making stuff get worse. Fun templates include gelatinous monster (Savage Species), bone creature, corpse creature, corrupted creature(all three from Book of Vile Darkness), symbiotic (Savage Species), multi-headed (Savage Species), ghost brute (Libris Mortis), horrid (ECS), and just about anything else that could really screw up an otherwise perfectly happy animal. And yeah, grafting undead body parts onto a bear or something would also get the PC's attention.