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...
My 2 cents. That shield design is a classic look. Divided into 4 spaces, and yet there are only 3 guardian classes. I think that blank space is intentionally blank. There won't be a 4th class. It's always been 3. Maybe blank space represents the others that aren't guardian classes. Or maybe it's just blank because the Vanguard is represented by 3 different classes. One of them passed away though and is not represented with a leader of their class.
Vanguard has always been 3 represented by Zavala, Ikora, and Cayde. That's what it has always been for 7 years now. Just my thoughts.