Just an idea, but how's the idea of earning points in the arena for defending, in addition to having the hero's defence buffed(if you didn't already know)?
As of now, mages and healers simply waste their turns when their skill is on cooldown. Even doing pathetically little damage to the opponent's heroes might backfire as their attacks might proc heal auras and heal them instead.
If you like the idea but don't want it to be abused, here are a few points we can discuss on.
1. Number of points earned
2. Not available if already on point-earning tiles
3. Number of times heroes can "Defend" per cycle (of 5 turns)
Will edit to include points that could be discussed.