I like the idea of Marauders having some sort of "occupation" that can give them the same access to racial picks, crystals, etc. Killing guards in capitals seems to fit their RP quite well. I'd just say when a marauder did these missions they'd have to be automatically attackable by other players (or at least players civ aligned to that capital). But I think that was kind of implied already in the first post, just putting it out there. Players killing the marauders that way should earn pvp or civ points as well, otherwise there would be no incentive for stopping them.
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"We are Kami. We are here to be you. We are many as you are of many minds. We are one as you are one in Ma-Duk."