Let me say up front that I doubt this could be implemented at this stage, and maybe this has come up somewhere and I missed it. But anywho, I had an idea about a simple shield/force field mechanic that could make for some interesting play.
Say your character has the power "Force Field." A single click on the power activates a personal force field that surrounds the player like a second skin. HOLDING the button down expands the field in a sphere around the character, but with a (skewed) logarithmic decline in power. So the "second skin" personal field is at 100% of it's damage absorption rate, but as it expands the amount of damage that gets through goes up. As long as you hold the button the "bubble" expands slowly (over a few seconds) until it reaches max size. BUT, you can stop its expansion anywhere in between by letting go the button.
This way a player could put up a bubble around his whole team and shield them all a bit, or just the tank, or just save their own skin (so to speak.) This could work either as a straight "absorb X% of damage" or "reduce to hit by X%." The expansion would continue out until it reached 0%. The range, as well as the percentage curve, would need testing and tweaking and I'm not qualified to discuss the necessary math. :)
A player could slot to increase it's size or damage, I'd think. Or it might be easier to simply increase the overall effectiveness and make one a function of the other.
Whaddya think? Is this an old idea that got beat down already? Would this be too "twitchy" for some folks?