Me and a few friends had an idea about this about the time Ttk came out we created something similar except we called it rouge guardians where you could choose to be s guardian or a fallen because they where a group of fallen who wanted to make peace with the guardians but the tower rejected them and any guardians who stood with them. That was until things got worse now they are welcomed in times of need. We chose their ability to summon other fallen to fight with them depending on the class. You could summon a captain or a survitor or an invisable sniper. That's about as far as we got before we got bored.