Stupid idea i guess but i thought it would be fun to think about a 4th subclass: Ice.
For example an ice worlock "Frost sage" Charged melee slows targets movement and reload speed by 40% for 5-6 second, Grenade that does AOE damage and slowly eats away at the targets super energy for a few seconds and super allows you to freeze targets within a few feet of you. Any damage taken while target is frozen will shatter them+kill them but they can thaw out in a few seconds or unfreeze if a friendly hits frozen comrades with any solar sources.
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If Bungie introduces a new element then we will probably get a new subclass. Ice or Acid would be great to have. The only disadvantage Ice has is that it might look similiar to Arc which could be confusing.