Maybe also add 'cast misdirection to self when defending' for Guardian's fortification. It's useless in Arena, but can be a gem in PVE content, as that way guardians can maintain aggro from the get go at the cost of lowering their dps (which is already abysmal in the first place). They can then aid in attacking after their innate taunt skill activates.
In fact, maybe it'll be much better to make additional effect when defending to be available from the start.
Mystic: reduce skill cooldown when defending (fortification talent may improve skill reduction further)
Guardian: cast misdirection on self while defending
Shadow: gain 15% dodge while defending
Archer: gain 10% attack and 5% critical for 1 turn after defending
Champion: reflect 15% of damage taken while defending.
Healer: reduce skill cooldown when defending
This, once again, to address the problem where mystics, guardians, and healer, tend to be only useful on occasional time and certain condition. I believe most of this suggested feature won't tilt the balance in arena as AI barely ever defended.