Rock/Paper/Scissors...
It would be a significant boon to the game if each toon were allotted a 20% inherent res/weakness to their power choice and it's opposing element; fire/cold and vice versa, neg/nrg, smash/psi, lethal/toxic.
Yes, many tanks would have a "weakness" but where the game currently sits, this would make for more challenging and involving gameplay.
Obviously the balance in game isn't equal and most damage comes in different forms, but this could be partially negated by making any form of say, smashing damage, follow this rule.
Lethal would benefit the most, as toxic damage isn't common, but inherent res to lethal damage is already throughout the game, so users are already limited on respect.
The proposed idea would work better if it was elemental fire/ice, water/earth... but this would involve too many mechanic tweaks.