This is a suggestion on a much smaller scope than my previous MMO ideas, but I was thinking about it over the weekend as a neat game play element that could be added to almost all the types of MMO's I've proposed so far as well as most existing MMO's and it seemed like a fun spin on a raid encounter.
I mentioned in my raid based MMO suggestion the idea of having the melee protect the casters by blocking for them. In this version of that encounter, the mages need to be protected because they are generating a shield that holds back the onslaught on an attacking army. The shield blocks all incoming projectile and magic attacks from the attacking army but requires the full concentration of all the spell casters in the party. The melee would have to go out and quietly and methodically take down the opposing archers and mages, while leaving some tanks behind to deal with the ground troops, who can walk through the shield.
The shield itself can take damage, so will require the full concentration of the casters to keep up. I imagine something like a "Simon Says" type game where you have to cast any of 4 or 5 different spells in an increasingly long sequence (the longer the sequence, the more power to the shield) until you fail and have to start over again. This way, the player would have to focus and would genuinely be distracted if enemy troops are getting too close.
Anyway, just a simple thought, but it seemed like it could be a fun twist on a tried and true formula.