In Lore we have heard about guardians that have not seen eye to eye with the vanguard. We know about the faction wars.
Why not add something like Dark Guardians, something in the game with hard as hell AI?
Who else would like to see rogue guardians?
We could use a Sith to our Jedi.
Or if you are a 40K fan
A Chaos to our Space Marines
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do it one better, allow players to become rogue if they want to and enable pvp in the open world, where the rogues can kill the normal players, and people could get bounties and shit by killing normal players and then rogue players can hunt them down and shit, and get rewards and shit if they manage to find and kill them.