This one isn't so much hidden, but hey, when you've got a series title, you gotta stick with it. Unlike with the Church of Law which has been obscured by tme, I think people have a good idea of what Druidism in D&D is: it's neutral and associated with Nature. In the Greyhawk setting and other places it's the "Old Faith" standing in perhaps for pre-Christian beliefs of Europe but without the Christianity.
I think there's another way to go, though, completely consistent with what the original works tell us about druids.
Druids first show up as monsters in the Greyhawk supplement. We are told they are "priests of a neutral-type religion." They can shape change and attractive barbarian followers.
They become a class in Eldritch Wizardry where they are described again as Neutral and "are more closely attuned to Nature, serving as its priests rather than serving some other deity." Mistletoe is holy to them, and they protect plants and animals.
Neutral may well just have been meant to imply unaligned here--not taking a side in the conflict between the civilizing force of law and the destructive forces of chaos: "I am not altogether on anybody’s side, because nobody is altogether on my side, if you understand me: nobody cares for the woods as I care for them," as Treebeard would have it. But maybe it's not just the woods the druid cares about?
Unlike Law and Chaos which seem to be transcendent and come from extraplanar forces, maybe Nature in this context is the cycles and balance of the material world? Given the description in Eldritch Wizardry, it seems likely to me that the religion of the druids is pantheistic with Nature (or the material plane) being an immanent divine force or deity. It could be animistic with everything in the natural world having a separate spirit, but it might also be monist, where divine Nature is the only true reality.
I think then that the druid's neutrality is a somewhat militant sort. The dualism of Law vs. Chaos is contrary to their understanding of the unity of all things; the strong, opposing polarities are nonsensical if existence is governed by cycles. Worse, these ideas from the Outer Planes would be alien intrusions on the harmony of the world.