The third sub class slot is the skill tree for space combat (Destiny)

by Kahzgul, Tuesday, April 14, 2015, 18:47 (3510 days ago) @ Dan de Board

I hope.

Seriously though, there are multitudes of options for skills that aren't currently in the game:

Illusionist/Decoy class
Trapper / freeze the enemies class
Direct support class (shooting allies charges them up/heals them/makes them immune to damage etc)
Necromancer class (raising the fallen enemies... fallen Fallen?.. to fight alongside you)
Vengeance class (damage reflection and redirection)
Posession/Meat puppet class (take control of an enemy and wear its skin for a while)
Suppression support class (doesn't deal much damage, but staggers and blinds enemies all day)
Close Quarters Combat class (takes less damage the closer he is to an enemy)
Hacker/mechromancer class (takes control of machine enemies)
Engineer class (turrets etc)
Goliath class (slow moving but has loads of health and deals high damage).
Gymnast class (very agile class that deals more damage when attacking from above and / or behind an enemy)

These are just off the top of my head with no actual thought or planning. I'm sure Bungie can come up with more.