(I'm still off work ill, understimulated and have nobody to bounce ideas off really so now I am going to ramble for far too long about a thing I am noodling around. Sorry.)
I've been toying around with GayHalforc's Sledgehammer rules, which I like for a bunch of reasons but mostly the fact that it's a rules light hack of Dark Heresy, which is a foundational text for a whole bunch of designers (myself included) who looked at its combination of low power, high lethality and lovingly crafted cross section of the Warhammer 40,000 and asked the question: What if this had rules that weren't shit?
If you want to hack Dark Heresy then Sledgehammer is a good place to start. It actually leans closer to Warhammer Fantasy Roleplay (WHFRP) but a simple reskin can set it in the 41st millennium and the rules are hacked right the way down so you can start reintroducing the crunch to a level that suits your own particular tastes.
For me this meant adding weapon traits (my group loves guns) and cribbing in Gamma World 7e's ammunition system (if you shoot more than once keep shooting cause you'll be out of ammo at the end of the encounter anyway), which is a brilliant rule I've always wanted to play with.
The other big change I wanted to make is more foundational; getting rid of the d% system.
It's something I've always wanted to do with Dark Heresy because percentiles get very 'mathsy' quickly, something made worse in the original system having variable bonuses and penalties depending on the situation. If you're shooting you +20 if firing full auto, +10 if aiming for a half action but +20 instead if aiming for a full turn. God only knows how lighting works, and thats before you even get to hit locations, cover and damage (Dice and weapon strength, minus target toughness, and maybe armour less the penetration of the weapon? ICK!)
None of which would be an issue if you didn't have to engage with that system, but stats in Dark Heresy are 25-40% range unmodified so you need to navigate all the variables to do anything reliably. Sure routine tests are supposed to get bonuses, but if they're routine why are we rolling at all?
I replaced all that with 1d10 roll under a stat between 2 and 5 (at base) which seems harsh until you throw in an extra 1d10 for having a relevant background detail and another 1d10 for anything that makes life easier. Anything under is a success, the highest of them is used for causing damage.
On the flip side particularly difficult tasks will steal a success after rolling, because that's how Heart: The City Beneath managed its difficulty and I found it suitably vicious for a hard edged setting.
Which brings us to the setting. One of the big problems with Dark Heresy's successor, Wrath & Glory, was that it tried to encompass the entirety of the 40k setting with its rules. Dark Heresy and its successors (Rogue Trader, Deathwatch, not so much Black Crusade) benefitted from focusing the action to a specific corner of the Imperium and the street level 'Cthulhu and boltguns' vibe of the original is definitely what I want to capture.
A big source of inspiration for playing Dark Heresy recently has been Warhammer 40,000 Darktide, which has a similar sort of vibe of picking your way through a massive hive city as the whole place goes to fuck. I'm not sure that I want it to be a full on warzone the way the Hive is there, but the idea of a place where Imperial rule is less absolute, rebellion more open; of a world straining at the shackles of distant masters and the space between the oppression of the past and the open rebellion of tomorrow, is something that appeals to me for reasons that you might recognise yourself.
So that is where my players will find themselves, with a name, an accusation, all of the responsibility and none of the authority needed to bring their target to justice and completely disposable to their distant masters.