One area that seems to be neglected in a lot of fantasy RPGs is the link between herbs and magic. The history of magic in our world teems with plants, both as magical objects or entities in their own right and as components necessary to creating magic. Something I was working on a while ago was a Fantasy Herbal based on the Iroquois system of medicine, which is basically a combination of herbal knowledge and ritual practice that imbues magical properties in plants if they are harvested in a particular fashion. I may try to polish it up and post it on this blog soon.
What I would like to see more of is plants as magic items. This makes the natural world that surrounds the characters a much more tangible thing - knowing the name, appearance and function of plants in the woods, being excited when you find a particular kind of flower, and knowing the ritual phrase to speak while harvesting it so that it becomes a ward against undead - that is the kind of stuff I am looking for. It makes seasons much more important, as they will determine what kinds of magical items are available to harvest. It provides great low level rewards (an herbal in the dungeon library that details uses for 6 plants is basically a recipe for magical items!). It is a built in adventure hook, requiring questing through the woods - I have the beginnings of a random chart that you roll on that includes both random monster encounters and random plants that you find in the woods, so an expedition to find Achillea Millefolium (yarrow), which can cure paralysis if the correct phrase is spoken while harvesting, an offering of tobacco is laid at the first plant seen, and subsequent plants are harvested in two's or four's, becomes its own little mini adventure.
I will be including a LOT of this sort of stuff in my upcoming Empire of the Petal Throne campaign - mycology and herbal knowledge will be very important, and natural products will be the majority of the low level magical items that the players will have access to. Mushrooms and herbs will take the place of healing potions, minor magical buffs like rings of protection, potions of flight, giant strength, invisibility, etc. This will involve trial and error or making contacts with knowledgeable locals, as the players will be barbarians newly arrived to Jakalla and ignorant of the local flora, fauna and customs.
A related subject to plants as magical items is plants as spell components. I have been finishing up an alternate magical system that allows for a magic user to create spells on the spot, without being more powerful than the standard caster types. One aspect of this is that components related to the desired spell effects make the spell consume less of your magical energy. Different plants are good for different broad categories of magic, so having a pouch full of different herbs would enable a caster to more easily create desired magical effects.
More on this later - now I must eat before the weekly 4e session!