A bit more elaboration on the Zodiac Guard and ideas for it: rather than a designated Librarius, the vast majority of the Legion is psychic to varying degrees, and aside from the most senior members like the command staff and other super-Chapter groups such as the Honour Guard, named after Ophiuchus, each Chapter in the Legion is named after one of the Zodiac symbols, with a particular colour defining it aside from the Legion's colours, and roles vaguely similar to the Zodiac they're based off of. For instance, the Sagittarius Chapter might be focused largely on jetbikes and ranged attacks; Pisces Chapter might focus on extreme environments, such as underwater or in the void of space; and at least one Chapter would be composed of Techmarines, I'm thinking Scorpius or Capricorn.
The exception is Cancer Chapter, which is where the non-psykers of the Legion are placed by default, and is ironically the only one which suffers any significant Chaos corruption, with a good hefty chunk of their members becoming tumor-riddled scions of Nurgle (yes, it's unoriginal, berate me all you want) that are damn near impossible to kill. After that failure, Cancer's defining colour switches from lime green to blood red, something else is done that forever mars the Chapter relative to their brethren, and- if you'll allow it,
@ClocktowerEchos- the Legion might start requiring the members of Cancer Chapter to undergo possession events akin to what the Exorcists Chapter in canon does, thus ensuring that all of its Marines are sufficiently resistant to Chaos to prevent further corruption.
