I do admit it'd be very cool, but we should avoid adding units for the sake of it. If there's a niche that's unfilled, or a new concept that's unexplored, unique, and innovative, we should definitely pursue it, but first we need to find what it's 
filling what it explores that hasn't been done, and so far, i'm pretty satisfied with the way it is. Edit: Point being that balance is fragile, and new units without meaning can make the game feel bloated, so we should be inventive with new weapons and abilities, or new combinations of stats to mix up the game.