I know the game isn't out yet, but from my experience I assume after you've mastered each class of guardian and all of their sub-classes things may get a little old, but of course this will take around 2 years depending on how much you play. Any chance of completely new types of guardians emerging from hiding in future DLC? If not then what about at least new sub classes to play around with for the Titan, Hunter and Warlock? Anyway I am fine with how the game is now, I'm just throwing out some business ideas Bungie!
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One of the DLC or some update that is possibly even free will contain another subclass for each guardian type (there's an extra slot if you hover over them in your menu) I saw a post or article somewhere that showed speculation of the new subclasses somewhere unsure but of you google it I'm sure you'll find something on the 3rd subclass :-)