I dont know if the story could ever allowed it and I know id rather have more content over skins. Just feel like this game is very skin heavy so why not include some more 40k trooper related. Would you other psykers love this or hate it? Let me know in the comments how wrong I am.
Librarians are space marines, so very unlikely to the point of impossibility. There are some cosmetics that look vaguely like the non-powered armour versions of their null hoods, which is probably the closest we’ll get.