Has anyone ever noticed that when you look at the subclasses there's an empty slot? It's possible their adding in subclasses for elements we don't have yet. If they do... These are going to be added with a DLC:
ArcThrower
Type: Arc
Class: Warlock
This subclass's super allows warlocks to create a sphere of arc energy that fires lighting at enemies within proximity.
Crusader
Type: Solar
Class: Titan
This titan super allows them to fire a pillar of solar energy straight ahead of them and is able to penetrate walls.
Trapper
Type: Void
Class: Hunter
Void subclasses are usually based on defense or attack. However this on is unique in its ability to be placed same as Ward of Dawn, but restrict enemy movement if touched (Could be useful against Bosses).
Hopefully this is implemented by Bungie in this coming DLC. See you in the Dark Below!
Disclaimer
I did not find this out; thanks to the community I knew about this. Thank you whoever found this out! This is something to look forward to!