The third sub class slot was there from when you were supposed to be able to 'lock in' a sub class, nothing else unfortunately
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Which is why that slot is still in game, today, 9 months later... It's also why each class is conveniently missing one damage type... Comet/Taken King will drop with a third subclass, mark my words.
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In the grimiore is was stated that titans couldn't wield solar light. So... (Not sure what card, but I have heard it in multiple posts and posters)
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It might, but that's nothing to do with why the space is there now.
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Bump This got removed after the beta. It was how you could max out the main stat of your class.