. Interesting concept. Let's start with the most basic implementation approach. You start an quest (like the legend of Acruis) where you get infected by the vex, aka like fail-safe's captain . One of those steps is to, say, take out 300 guardians, the reward at the end of the entire quest a bad ass evil weapon that was banned by the traveller. Similar to pvp they would show up red on the radar and warning messages would appear stating 'vexed brained washed' guardians are roaming the planet. The "evil" guardians roam the planet and take on other guardians (only public places and not strikes etc) .. The normal planet spawn points are disabled etc to make sure ppl don't just camp at a location. Just shooting from hip and I am sure others could expand on it. Could be fun though. Can you imagine a public event fighting not only the cabal but rouge guardians as well.