They should have a seperate resource for defensive abilities like dodge roll, blocking, break free, shields, defensive buffs, and healing. Then the armor types can give bonuses to their individual defensive type. So light armor would give bonuses to the defense resource associated with healing and damage shields. Medium armor would give bonuses to the defense resource associated with dodge rolling and break free. Heavy armor would give bonuses to the defensive resource associated with break free and blocking.
This would further separate the healer, dps, and tank roles. Tanks and healers would focus on the defensive resource where dps would focus on their individual offensive resource.
In PVP you would have to decide between all damage, all defense, or a balance between the two. There would be no builds that have both high damage and defense.