I've been pretty up-front about some of my pseudo-borrowings that I've adapted into my setting. One such is the notion of a very wild, dangerous and frightening approach to nature, as opposed to the hippy silliness that pervades many fantasy campaigns. This may seem odd coming from a guy like me, where one of my main hobbies is wilderness backpacking, and I absolutely love spending time deep in the remote Rockies. Then again, I've been exposed to dreadfully frightening inclement weather—heat in the desert in the summer of West Texas and southern Utah. Hailstorms when there's no shelter to be had above 10,000 feet in the mountains. The need to come down off of ridges and peaks as storms roll-through because lightning strikes are suddenly no longer sufficiently statistically anomalous to take that chance. I've hung bear bags to make sure that they don't disturb my campsite at night. I've hiked for miles wondering if my next water source was going to be there, or usable, when I got there. I've even limped back to my parked car, defeated, exhausted and frustrated in my designs before, due to minor injury, exhaustion, weather, or just plain "I'm done."
woodwose, or wild man. The Dark Forest inhabited by savage, hairy feral near-humans has been hypothesized by some, at least, to be an ancient racial memory of anthropophagic Neanderthals. Nice idea. Great story potential in that.
Even before I'd heard of the Neanderthal Predation Theory, I'd been attracted to wild man archetypes and rules for such. I'd early made the easy choice to adopt shifters from Eberron into my setting, and I later adopted Neanderthals from Frostburn in as well (back when my setting was more overtly d20.) These Neanderthals weren't the predatory primate version of them, but the more mainstream very human-like interpretation. But I gradually started getting a darker and darker vibe towards this whole idea. Incorporating a lot of the vibe of the Tharn and other worshipers of the Devourer Wurm from Iron Kingdoms, and a lot of stuff from the Beastmen army of Warhammer, I ended up with woses, and thurses, rather than shifters and beastmen, but the concepts are similar. The woses are more likely to be hospitable, at least among some populations, but I wouldn't ever count on it for others. They are still a major problem in the Shifting Forest, at least.
But what I really want to add is the notion of incredibly hostile human peoples living among them; maybe even ruling them, as druids. I don't imagine druids as being the pseudo-hippy new age clap-trap variety of druid. I imagine druids as being almost feral creatures who run naked through the woods, kill (and eat) trespassers with their bare hands and teeth. run in packs not unlike wolves (and occasionally gather for various reasons, not unlike grizzlies at a salmon run) who are so completely and thoroughly hostile to any form of civilization that they make the massacre of the XVII, XVIII and XIX legions look like a friendly rebuke.
The druids may (as in the Circle Orboros faction of Hordes) recruit monstrous wildlife of the primal wilderness to serve them in times of need, as well as chthonic spirits made flesh. They also handle extremely powerful (albeit often subtle) earth magic, all of which tends to be great and terrible; more akin to the tornado, flash flood, or forest fire than the more pin-point, localized magic of sorcerers and witches. Remember; this is dark fantasy, and my interpretation of dark fantasy is somewhat like classic fantasy that features the tone (and plots and many of the conventions) of a classic horror story. What you would get if Dracula, The Wolfman, or Woman in Black were set in Middle-earth, so to speak. So if you make your druids more like Beorn than like R. A. Salvatore's Mielikki worshipping neo-fantasy-hippies, and then make him even darker and scarier, you'd be on the right track. It's maybe more a combination of Pan's Labyrinth + The Wolfman + a bunch of feral people stories.
I would use the "expansion rules" for m20 (the ones called Microlite20 Expert Rules) with the Druid class; but I wouldn't allow players to use it. Druids, by definition are fey-touched, or wild-touched—insane, in other words—and serve only as militantly hostile and bizarre, scary opposition.