Note that after you unlock the second subclass for one character, it will be unlocked for any new character of the same class that you make in the future.
Striker
Defender
Crusader
Crusader is a rumored Titan subclass that will come with Comet: Plague of Darkness.
Gunslinger
Gunslinger is a Hunter subclass that players start off with.
Bladedancer
Bladedancer is a Hunter subclass that players unlock at level 15.
The Trapper
The Trapper is a rumored Hunter subclass that will come with Comet: Plague of Darkness.
Voidwalker
Voidwalker is a Warlock subclass that players start off with.
Sunsinger
Arc Thrower
Arc Thrower is a rumored Warlock subclass that will come with Comet: Plague of Darkness.