One of my biggest beefs in games these days that involve magic is always one of the four prime elements, air. Specifically, the method of attack associated best with air, lightning. It always seems so underutilized in games these days to find any form of spell caster that uses electrical attacks (hell, air in general) to get the job done, and I believe an air elementalist style demigod assassin would be a great addition to the lineup (perhaps an air/earth elemental style to have opposing elements much like the Torchbearer's Fire/Ice).
I'm not really good at the whole art design aspect, so how the demigod looks is open to interpretation, but specific attacks could range from:
AoE knockbacks, potentially knocking minions off of cliffs (this would be fun). targeted lightning strikes with perhaps a knockback or a stun, cyclonic roots that toss people. I can't really come up with more balanced ideas due to the fact that I don't know what abilities currently exist to great detail, and don't know how to mix/match them properly, but those are some ideas.
Others would be chain lightnings and anything else cliche. But, most importanly, lightning, it has to exist on a demigod!
If an earth/air elemental assassin were used for the opposing elements thing, then you can start to mix in a "quicksand" like ability for AoE snares, you can chuck giant rocks at the enemy pulled up out of the ground around the assassin, sink (minions) into the ground and a lot of other fun stuff.
But, the original point, essentially, is I'd like to see more underutilized elements as viabilities, since fire and ice seem all the rage these days, but lightning seems to be the red-headed stepchild and, let's face it, lightning is awesome.