I think the best way to deal with this is to get away entirely from the traditional tank/healer/dps roles. That trinity doesn't really fit in a sci fi MMO.Chairman Miaow said:
Much as I hate to draw the comparison, look at a class-based shooter like Monday Night Combat. For that matter, look at the classes Mass Effect already has. Everyone has his particular role to play, and they don't really fit neatly into the Big Three. This would be a great way to introduce different kinds of party structures, and would be a welcome change from the simple tank, spank, and heal.
You could have light recon, snipers, heavy weapons, biotics specialists, tech specialists, combat medics, etc. Everyone would have to be able to deal with enemy fire in some way, whether that's through heavy armour, agile dodging, biotic shields, relying on staying unseen, or even simply taking cover. It'd be a very different way to game than WoW, which I think would be another big point in it's favour.
