Same regular SFX rules:
-Ingame textures only
-No bigger than a 512x512 square in the editor
-No more than 1000 particles on one SFX
-(and all the regular contest and site rules)
-Nature would be your typical RPG element: grass, earth, trees, poison (maybe), roots and all that stuff.
BTW, I just thought of this... Death (and/or Demonic) would be a cool theme too. In the sense of blood SFX, undead magic and bones, demon energy, etc... Mayb for the next contest or something.