The Cabal could even reach out to the guardians, since they don't have any real reason to fight us, they're merchants and warriors. How about the darkness comes to the cabal homeworld and since we've beaten both crota and oryx they hire guardians as mercenaries to help them fight the darkness. Some of the legions could disagree so that we still have to fight cabal, but some of them could ally themselves with us, I think it could be really great :)