I think that all mechanics based on "You have 15% for something" are wrong. It is random effect which Air didn't need. And what is it 15%? We have a lot of discuss about problematic 40 and 50% in shields and we didn't need next percentage chaos in game. What is more - 15% is very very low. You need 3 SoFr to get "normal" chance to see real card effect in game.
I prefer versions when for example 5th creature gain effect (it is like 15% chance for all creatures, because we must remember about 0% chance until we have <5 creatures). Promoting big number of creatures are also compatibile with Air (this element has got small and fast creatures).
I thought also about something like this "Creatures with 2 airborne neighbours on field are unreachable for opponent. Air creatures avoid shield". Here when we want destroy creature we must earlier hit his neigbour on field. And special bonus for Air - avoid shield (no, it isn't OP, simple Explosion give also "avoid shield" effect for all elements and cards in game, here only works with Air). Then it will be real Freedom, real good card for teamwork with airborne and with bonus for Air (soft PC like shield avoid is something what Air need)