Since all my week is evaporating like paint thinner  in the summer breeze…  I figure this will give me leeway to have  fantasy and sci-fantasy terrain and make up some Mordheim-style rules to  go with the Necromunda-style territory acquisition thing I like.  More  an academic exercise, although I managed to include my Imperium troops  and give myself a reason to get a squad of Tau, Eldar, Squats, and  Kroot, and maybe even some Dark Eldar (I really gave myself some wiggle  room!)
On  the fringes of the Segmentum Pacificus, far from modernizing  influences, lies the Orvisto system.  The fourth planet from that star  orbits just far enough from it to receive sufficient heat and light to  keep the human inhabitants uncomfortably chilly, but there is little in  the way of free water on the surface to make snow or frost.  It receives  most of its food from the nearby Catellaste VII, which is an abundant  producer of synthoy and vitamin pills.
In  its heyday, Orvisto IV was a librarium planet, and a minor factorum of  real books and holodata displays.  It even served as a repository for  manuscripts and prints from all over several systems during the last  Flood of Fire in which Imperial and Tau worlds were ravaged by Khornate  Marauders.  The nobles of nine different industrialized Human planets,  including their Tau and Eldar allies, all placed sacred books,  histories, and accounting ledgers onto slow barges to safeguard them  from the torch, and promptly forgot them when those planets were razed  and ravaged.  The end result being that mountains of rotting books and  stacks of discs tip over into the sparsely-inhabited byways of the hive  cities.  Most of the sensible folk have fled the system for better  pickings, leaving millions of foolhardy souls to pick over heaps of  worm-ridden and mouldy paper, looking for clues to riches buried  underneath in the lower levels of the hives.
The  inhabitants are at a late medieval level of subsistence, offer worship  to The Emperor of Man, and most know of and work with several minor  alien races.  Some are semi-literate and few are even very well-read,  although to proclaim such is dangerous.  These sorts of infractions are  overlooked owing to the delicate and valuable nature of the treasures  the planet holds, especially since the regulation and oversight of the  process is placed specifically in the hands of a Dark Angels successor  chapter, The Guardians of The Covenant.  Who or what ‘the Covenant’ was  is unknown to the common inhabitants of Orvisto IV, but it is assumed that it  involves the safe-keeping of mostly worthless words and ink and bytes,  to secure some of it (possibly very valuable) against loss.
Thus  the Battle Brothers of Orvisto IV, Guardians of the Covenant, in some  sense rule the hapless planet.  There is an Imperial Governor, Hew-Watt  by name.  He is a wizened cyborg, charged with administering to a  machine in which men and women are cogs.  The human population of the  planet is mostly devoted to uncovering, sorting, moving, and (rarely)  translating texts, and piling newly received ones in a way so as not to  disturb lower layers.  This slow accretion of words and paper and  silicon has dangerous consequences, in that paper can catch fire, can  house molds and insects, and every so often men and women are buried  beneath avalanches of old magazines and encyclopedia.  The Guardians of  The Covenant enjoy good relations with nearby Mechanicum enclaves and  send recovered specifications and rumors Mars-ward whenever possible,  and the great Librarium Fortress of Spire Contina has regiments of  text-scanning and –processing servitors that are maintained in good  repair.  Much of the technology in use is thousands of years old and  well-maintained although prone to machine-spirit indolence.
There  is a contingent of Adeptus Sororites as well, the Grey Ladies, charged  with maintaining the spiritual purity of the populace.  It happens that  the digger gangs and reading-and-calligraphy guilds (the so called Copy  Clans) will rarely find great caches of questionable reading material  from some far-away (now destroyed!) planet and although they often have  no idea what the texts say, they sometimes are responsible for the  spread of objectionable ideas and heresies which the Grey Ladies are  happy to suppress.  They are especially watchful for Dark Eldar  insurgencies, since that race is drawn here to raid for docile servants  and pliant torture-victims.
The  Planetary Defense Force is garbed in grey of an antique sort, with  crudely burnished helms and armor after the fashion of their respected  benefactors.  They have access to old and sometimes dangerous  technologies, by virtue of long-covered store rooms and occasional  archaeotech hoards.  They prefer to use autoguns and bolters rather than  laser-weapons to minimize risk of Stack Conflagrations.  But some  squads put flamers and explosives to good use in quick destruction of  suspect information.
The  local Arbites forces wear similar grey carapace armor and sometimes put  down synthsoy riots and patrol The Stacks for xenos scum.
There  are the usual percentages of abhumans – many Copy Clans employ ratlings  for seeking out texts and tech in tight places, and Ogryn for heavy  lifting.
In  addition, there are very small groups of xenos that are permitted to  exist on Orvisto IV, mainly the Tau-allied species and even some actual  Tau agents (prohibited from proselytizing on pain of death), as well as  licensed Eldar traders and their associated races.  Kroot are known to  track down high-value knowledge in the mid-level warehouses, if they are  given sufficient clues and tattered maps.  The Battle Brothers and grey  Ladies overlook these influences and take care to present a kindly and  beneficent face to the standard citizen, but commoners who know too much  or mix too closely with xenos without dispensation are prone to  disappearing in the night, never to be heard from again.  Nobles and  clan-leaders who employ alien agents do so with the cool gaze of the  Guardians, and are monitored closely by several low-level Inquisition  agents.  This includes a handful of irascible and generally bored Ordo  Hereticus and Ordo Xenos workers , as well as a lone Ordo Sepulturum  Inquisitor – Valkrim Hax – who ensures no more outbreaks of Neuronal  Plague.  The world has not had a visit from an Ordo Malleus Inquisitor  in several centuries…
The  whole of the planet is divided into giant arcologies, the top levels  housing Imperial administrators and the more illustrious production  clans, and (as on other hive worlds), the bottoms of the arcologies are  steaming, festering masses of human and non-human filth.  Unlike  Necromunda, for example, the spaces outside the giant arcology-hives are  divided into clan-owned and –managed pulp forests, canal-farms in which  ink-squids are raised for ink and isinglass, and mills for making paper  and book-boards.  Towering spires of Calligraphy Guilds teeter and sway  in the wind.
Most  of the non-spire inhabitants subsist on birds, game, squid and fish, as  well as crude black bread from off-world and their own  hydroponically-produced vegetables.  The sight of the off-world bakery  ships in the sky is cause for great rejoicing, usually given a  feast-day!  Inside the stone and steel arcology spires, digger clans in  the midlevels trade synthsoy crackers of various colors, vita-drinks,  and whatever meager food stuffs work their way in from the dense forests  outside.  The lowest sump-level inhabitants are not averse to  cannibalism, but luckily they can survive on giant book-worm larvae,  helminths, and dire rats.  Power in the spire-vaults is a tricky  business, and sometimes water and steam access is questionable, also.   Beneath the arcologies’ production and habitation levels, they are  joined together by massive stone storage vaults, oubliette-networks, and  tunnel-webs that interlock the planet – these are generally just above  the sump levels and none still know the ways the dozens of spires still  interconnect.  Indeed, whole clans of Digger-Mapper Squats live and die  without ever seeing the Orvistan sunlight and may even go mad if they  spend too much time in the open air.
In  Gervog Spire, a small trading clan – formerly makers of luscious purple  ink – have expanded their claim on water and fragtose resources to  create the Taint Cola Company.  It was formerly only popular amongst  nobles in the various arcologies but it has eclipsed the Imperial  Standard Soft Drink in popularity amongst the citizenry at large within  the past few solar cycles.  The Clan Purpurens has always been low-key  and trustworthy and a steady supplier of pigment to artisans  sector-wide, but demand for their soda necessitates forceful acquisition  of water and fragtose rights that  historically belonged to other  clans…  Citizens await their soda shipments with an almost religious  (but not TOO religious) fervor and riots have broken out when supply  diminishes – a strange event amongst a normally bookish and complacent  populace.