could a new mechanic like this help?
When you swing a melee weapon, you get a defensive focus counter. Which would stack up to let's say 500.
Whenever you cast a spell or SLA, you lose some of your defensive focus (say 40 for a cocoon or 100 for a wail of the banshee).
Defensive focus would somehow increase your defense. Maybe a multiplier to your PRR/MRR?
Switching to a ranged/thrown weapon would lose all stacks.
Not sure how to make this compatible with tanks though...