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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The problem with making another enemy type is you need to have enough rogue guardians in missions to make it interesting. Lore-wise, there’s probably no more than 20 rogue guardians we know about. Maybe enough for one mission, but even then, it’s not worth designing, coding, or bug testing for the dev team.