So before we begin, this is on the possibility that we end up getting more darkness classes. Bungie may decide that we only get 1 and we don't get more. This is on the side of if they do want to have say 3 darkness classes. with that out of the way lets begin.
So my theory is that the second class would be Corrosive in nature like Acid. It being a class meant to deal damage over time rather than all at once or to slow down the push. Basically it would be similar the Hunter's tether where it slows and damages any caught in it.
Anyways that's my theory, feel free to add your own potential Elemental classes. I'm kinda stumped right now on what a third might be.