Just some suggestions regarding fire and poison:
Fire:
The Warrior skill "Helping Hand" should be usable to put out a hero on fire (in addition to it's normal use).
Gradually increasing intensity from fire damage to remove the feeling of gasoline covered heroes. For example..
1.Round: 15 dmg, 2.Round: 30 dmg, 3.Round: 45 dmg ...etc.
Poison:
An incredibly deadly kind of acid as it is implemented right now. Poisoned ground should not be abled to do damage until transformed into a poison cloud. It should act like water or oil until transformed.
A secondary effect like Constitution-Damage for the duration of the poisoning would also help to make it more unique and not just an ordinary dot-effect.