I never looked at them as subclasses because that specific term was already coined in the game according to them as being a single class that has a very unique theme already setup for it, like a shifter is a Druid who is uniquely focused on their spirit forms. My idea only focuses on the Primary classes involved in the combination. I'm not even sure why you'd even consider the subclasses in this case as they are purely optional.
Maybe add a unique skill for each class combination, so as to justify each one being considered its own class. Maybe a ability that blends the idea of the specific combination.