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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Why not make these rogue guardians a separate player faction? Like in WoW where there is Horde and Alliance.