So, what do we have plotwise so far?
IIRC its a rebellion on a hive world that (unbeknownst to the Imperials sent to quell it) is under the influence of chaos (or maybe xenos).
Trying to flesh that out, I think a mix of the
Vraks campaign from Imperial Armour and the
Verghast campaign from Gaunt's Ghosts works well.
1.) For whatever reason, the rebels/traitors need to have an advantage over the initial imperial forces trying to knock them back in line; the PDF would have all turned traitor (the loyal ones killed) - as might several regiments of IG proper that come from the world and were stationed on or around it when the rebellion starts, meaning the entire system might be in danger of being lost.
The Arbites would be dead (probably the first casualties along with any commissars and ecclissiastiary types)
The governor and his command guard would have either defected as well (maybe the heresy starts at the top?) or has been deposed in favor of some apostate priesthood preaching heresy, or a factory worker's revolt wanting independence from the imperium, or something similar would happen.
2.) Whatever Imperial forces were sent initially (one or two IG regiments from a nearby planet?) made little to no progress, and now the rebels control most of the world.
This affects the imperium because this particular hive world is a major manufacturer for the sector's IG regiments (also explains how the rebels can get ahold of better weapons and heavy equipment) and thus would hurt the imperium too much to simply "exterminatus" what appears to just be an insurrection by long-repressed factory workers and such.
3.) I would think the first guard regiments sent there would be those in the same sector/sub-sector; for example with a few exceptions (namely famous regiments like Cadians, Kreigers and Catachans that were probably sent there to provide a "core" of ultra-reliable troops),
nearly ALL the IG involved in the 3rd war for armageddon were from that sub-sector (the infamous
Savlar Chem Dogs for example were basically the resident penal colony of the sector and might have actually been formed specifically to fight wars against Orks on Armageddon and the nearby worlds).
That said, there could still be dozens of kinds of planets (
hive worlds,
feudal worlds,
agri-worlds,
death worlds, etc) it really gives people the freedom to come up with what kind of regiment they want, their skills and strengths/weaknesses and what gear they want them using - obviously within limits of what IG can have, but for example
hive world troops would use totally different equipment than what
feral worlders would other than the basic lasgun/flak vest.
4.) Most of the planet has already fallen to the traitors (possibly so have other less-important worlds in the system that are culturally close to the Hive World or garrisoned by its rebellious PDF elements - something like this happened in the Vraks campaign initially).
The players would either be what's left of the first wave sent to fight the rebellion, or be last-minute troops (probably
drop troops and the like) dropped into the remaining still-loyal hive(s) before the imperials lost air superiority.
These last guardsmen sent in - brought in on
lighter landing craft that are
more manueverable but
carry far less troops that the usual company-sized (IIRC) ones - would be tasked with the difficult/near-impossible task of spreading Imperial control back to the entire hive and "holding the line", so that a larger force of several regiments can be landed and conventional warfare can begin.
***
This basically means:
- A.) the IG have no idea what they're getting themselves into since they just think these are petty rebels, not pawns of chaos.
It only gets worse when mutants, chaos spawn and plague zombies start appearing, followed by the
resident Blood Pact prioxies (I'm hesitant to include chaos space marines so these will fill in as the "regular" chaos troops), daemons and whatnot.
Maybe xenos allies too; I know orks and even dark eldar have been known to ally with chaos forces, and there actually are some chaos-worshiping minor races like the hrud and loaxtl.
This is all compounded by the fact a good majority of the guardsmen don't even know what half the things they're fighting are, and were never trained to face anything but other humans and maybe certain xenos breeds or rebellious mutants.
- B.) Enemies can be anything from underhive gangers (ex. mutants known as "
skavies") with blunderbusses and laslocks using the war as an excuse to run amok, to troops
as well equipped as the guardsmen themselves are (sometimes better), and even occasionally Leman Russes or a FUCKING BANEBLADE! And that's before the chaos and xenos are factored in.
- C.) The guard are likely incredibly varied, and as casualties take their toll on units guardsmen from cultures some of which are practically alien to one-another must learn to cooperate to survive.
There's also the constant threat of desertion if things start going badly (especially with how charismatic the rebel leader seems to those who hear his rhetoric), so commissars and officers alike would basically be stressed to the breaking point.
- D.) The players' guardsmen are in over their heads, outnumbered and basically fighting as guerillas initially - but if they can succeed then the IG will be able to bring its full strength to bear; right now they can't because they lack air superiority and lets just also throw in that some unknown pirate types (renegade imperial ships ala
red corsairs? ork freeboota mercenaries? who knows) are helping the rebels combat the imperial navy so they have their hands full with fleet operations and can't use orbital bombardment or afford to deploy enough fighters and bombers to take control of the skies.
Maybe the rebels also have control of orbital defenses and can blast ships over a certain size out of the sky with ease (hence needing to infiltrate with valkyries and landers) which also explains why the fleet can't just sit up above them and blast away.
This initial goal could be what the first "arc" of the story is about; regaining control of one or two hives that have the remnants of the loyalists holed up in them.
Edit: I added some links.