Well I guess I'll voice my support for multi-ability skills, but it very much would require wholly different skills than we have currently. It would definitely require some kind of clear skill tree built in the game, or otherwise extremely intuitive ability trees and skills, which I think the OP's skilltrees are for the most part. Though I do think putting all elemental skills into the same tree is a bit much for how important elemental interactions are in the game (and presumably in a D:OS 3 as well).
Maybe you could have, like, the "Temperature" tree which deals with heat and cold, which is more the pure magic damage tree, and an Earth and Air tree (Paganism? Or more like manipulation of "Substance") which is more utility oriented with buffs, terrain manipulation, knockdowns, etc.
