Reddit Thread

It seems reasonable that they're going to include a third subclass which encompasses the last of the three elements. From what we've seen of the current classes, the elements have the following in common:

Arc: Melee Focus

Solar: Ranged Focus

Void: Utility/Control Focus

• Arc currently has Striker Titan and Bladedancer Hunter, two classes that are strong at melee range damage dealing.
  
• Solar currently has Gunslinger Hunter and Sunsinger Warlock, two classes which are inherently strong at dishing out high bursts of immediate damage as well as targeted DoT damage.
  
• Void currently has Voidwalker Warlock and Defender Titan, two classes which are strong at manipulating large AoE zones for offensive area denial or defensive fortification.
  

With these themes in mind, I'm currently speculating that in the near future we'll be seeing a Melee Focused Arc Warlock, a Stealth/Trap Focused Void Hunter and a DoT/Enemy Detonation Focused Solar Titan.