As I mentioned before, a basic environmental biology or ecology text can provide ideas for dozens, if not hundreds, of druid spells. I was flipping through a textbook and came upon symbiosis. Even though that idea hasn't been well used in d20, there is something much larger related to symbiosis that I think has more potential for designing druid spells. Symbiosis is a very generic term meaning a close relationship between two species. There are several kinds and the big three are parasites, commensalists (where one benefits and the other is unharmed) and mutalism (where both benefit). What is now being discovered is that those catagories are fluid in nature. A parasite can have some beneficial properties for its host (usually enhancing the immune system but there are many others) and mutalists may eat each other depending on the circumstances.

That idea of a fluid dynamic of nature is my point. Nature is inherently a battle between chaos (entropy) and order (biology). Life exists because of both, riding a wave that forces it to constantly adapt or die.

There are a few ways of using this in d20 and OSR games. The easiest is to have an environmental component applied to each spell. Spell X works best in biome Y. Everywhere else it is weaker and in some places it doesn't work at all (an idea that actually dates back to 1e- how casting underwater works). Metamagic feats work just fine in this regard. Some of the possible components can include weather, star configuration (stars are part of nature), monster types or specific species, terrain, night/day/twilight, elements and elemental portals.

A harder way to go about this is to replace the druid spell list with free form magic. There are several good sources out there for this. Even with freeform magic, there must be an environmental component for each casting or effect. The ambient magics that druids tap into wax and wane constantly. Ley lines are simply highways of magic and there are many more back roads of many kinds of magic. Multiple components can be used to make the desired effect even stronger but the danger is those places where the lines do not reach, thus making some spells useless.

The other way I thought of is one I haven't seen, a mixture of the above. Existing spells that can be altered on the fly in ways that are not related to power (and thus not metamagic feats). This is even harder as you have go through the entire spell list and come up variants of each. An example would be Barkskin that is influenced by the local vegetation and does more than just provide armor. It may allow photosynthesis, camoflauge, a bonus to diplomacy with plant creatures, allowing the caster to go unnoticed by some plant creatures, the ability to drink toxic water, etc. This throws many wrenches into d20 and may cause serious problems for other D&D based games. But I hope there are some of you willing to give it a shot, adopting it in some form.

The nature sense class ability (and in older games, the ability to identify plants, animals and water) is so much more important when using the concept of dynamic magic. The druid has to take stock of their surroundings all the time, otherwise the addition or loss of a component may impact their abilities in ways they may not understand until after a spell is cast.

Of course a druid has more supernatural abilities than spells. Wildshaping is the other big one and it too might be affected by the environment the druid is in.