I was thinking of mechanics that might make for an interesting blast set, and my thoughts wandered to the Dreamcast game "Ikaruga", where one of the core mechanics is that you can switch between dark and light shots, which grant immunity and extra or reduced damage to like/opposite types. To that end, I present "Polarity Blast":
The set would have a swap mechanic similar to dual pistols' swap ammo, but only between light and dark, (or negative/positive, etc). Attacking with one type would apply a special debuff to the target, and being hit by such an enemy while in the same or opposite mode, (the specifics can be ironed out later), would result in reduced damage or such from said enemy. Perhaps the set would also include a short but potent self buff for switching damage types, in order to encourage use of this mechanic, Other than the switching mechanic, the set could easily draw from existing attacks and animations. To simplify things, perhaps the 2 mutually exclusive mode-toggles could simply apply an additional graphical effect, so as to not have to deal with how to handle each attack having 2 separate FX themselves...