If you look at the vanguard class emblem, it's almost like it's missing something.
It has the titan, warlock, and hunter class emblems on them and then a 4th, almost just...space wasted slot.
Is Bungie just being lazy? Probably.
Perhaps it's just insurance in case the team decides to throw in a 4th class.
I like to think maybe they got something random in the back of their minds.
Food for thought...