Plant that is likely to produce allergic reactions in almost everybody, even by touch. Usage: medicinal, in very exotic foods, non-lethal traps, disabling opponents (have you ever cut an onion,anyway?), spell components, etc.
A local fungus-breed explodes if hit or fallen upon. If there is any conflict between inteligent enemies,both sides are likely to use it to their advantage.
Elves use nature as strength and borrow from the trees to keep them living. An elf out of the forest for more than a day begins to die and within 3 days is dead without borrowing the strength of nature.
A race that are always born as twins and only one recieves a soul. The other is soulless therefore without pity, shame, or guilt. The evil race. Can't tell them apart by looking at them.
A flower possibly in the plains or maybe all over that is always tilted towards a certain place. Sunflowers. Could possibly follow the sun or could be forever facing a certain position because of something that happened, either the air forcing it that way and it staying, the fire was so strong that the flowers faced away from it. It could be that in one spot a large fire was made or a fire war where many fire elementals were destroyed by the True Gods and radiating out from that point all of this flower is facing away from it so no matter where you are you always know where this place is by seeing which way the flower is facing away from.
Something lives in the fog in a certain region and travels with the fog. Kidnaps or kills, or is simply heard and "felt".
The two cathedrals in the Citadel stubbornly ring their bells two minutes apart, since each believes the other to be too early or too late respectively. This has been done for over two hundred years.