Thanks, everyone, for sharing your thoughts and campaign traditions. Here's a draft of what I'm thinking about for my current campaign world.
My current game cosmology is rooted in the loose system used in the Dungeon Fantasy Roleplaying Game (kinda "GURPS D&D"). Magic is divided into three realms: clerical, druidic, and wizardly (other powers can be based on inner strength, like a monk's chi abilities). Druidic magic comes from life itself. Like traditional D&D wizardly magic, druidic magic is not actively mediated by any conscious entities (at least not that the mortal world knows of). The morality of druids, therefore, is as wide-ranging as that of wizards. (Clerics have their deity to contend with if they stray too far from the expectations of their sect.)
In some regions, druidic orders have developed (like wizard guilds) that enforce codes of behavior. They may actively pursue "rogue druids" to maintain their reputation (or doctrinal purity, or their monopoly on nature power, etc.). Such a druid society may be fully woven into the society and politics of a region. In other areas, druids are more solitary or form competing circles. Depending on the area's particular history, they may be mistrusted or seen as primitive and uncivilized. It's even possible that in some areas they are actively opposed, outlawed, and/or hunted. Because druids who stray from any particular moral path will not be stripped of their powers, even independent druids may have an incentive to eliminate brethren who make too much of a nuisance of themselves. In D&D terms, druidic morality is similar to the default 5e setup where they can be of any alignment, though something akin to neutral may be a common choice.
Druidic magic works best in the wilderness. It is weaker in artificial areas like cities, castles, or dungeons. (There's a sliding scale). It is at its worst in areas that are truly devoid or toxic to life itself (perhaps places defiled by magic, like the old Sea of Dust in the WoG). This suggests that it wouldn't make sense for an "evil" druid to be literally opposed to all life. They might oppose particular aspects of life (civilization, wizards, hordes of humanoids, etc.), but they wouldn't be creating radioactive wastelands (unlike, say, a particularly depraved wizard). Similarly, undeath is antithetical to life, so they wouldn't normally ally with necromancers and whatnot. Though I could see druids who have no problem animating natural corpses via unorthodox methods (fungal infestations, animated vine exoskeletons, yellow musk zombies, etc.).
Alliances with Lovecraftian horrors seems similarly unlikely, though I'm not ruling out the "different nature" idea I mentioned in the OP.
I'm still mulling over the "old religion" stuff. I have some work to do figuring out the Big Picture theological questions of the campaign world. I'm also unsure how elemental magic fits into things. In default DFRPG, druids can wield elemental magic because the raw elements are "natural." I like the idea of a Volcano Druid or something like that. An erupting volcano is obviously natural, but it's also pretty toxic to life in the short term, so I'm not sure how to balance that equation. I recognize that eventually, new life will emerge, but it feels qualitatively different from a disease epidemic or plague of locusts. In those cases, there is death for some creatures, but plenty of other life happening at the same time. A massive lava flow pretty well wipes things out until life can creep back in.
I'm open to further stories from your worlds, or questions, suggestions, or critiques of this sketch.
