No word yet.
In earler versions of the game, you used to be able to "lock" in your class ability choices in return for a ability bonus. It was speculated that third spot was for a new subclass template that level up faster. (IOW, like a second Voidwalker template if you're a Warlock). So you could hve multiple loadouts
But now now one really has any idea (yet) what that extra slot is for now that the locking function was removed.
...and Bungie isn't telling.