Arbites and gangers would be cool, maybe they could defend whatever passes for a courthouse in WH40k though maybe that makes less sense against Tyranids rather than heretics unless it's genestealers.
This one makes surprising sense, since 40k courthouses are usually covered in guns and are basically bunkers. Wouldn't be much of a stretch for an arbites chief to look at the wall of space bugs and decide "it's time to open the doors of justice" to the underhive as criminals loyal to the Emperor are all sacred in the eyes of necessity
Alternatively, the arbites are mid-way through another crime crackdown when the space bugs attack. Arbites and scum both reach a tacit understanding of "let's not kill each other today" and begin fortifying the underhive until reinforcements can arrive
Khornate berserkers vs Tau would be neat if it was like an endless dungeon where they keep plowing through until they get stopped. Or they could reach the boss fight location, then they control the other group raging towards their objective and see if either group makes it there. Each team could have a mini-boss organizing the defenses they could defeat or bypass at the cost of the mini-boss helping in the boss fight. I guess it would make sense to narrate there are other groups not under player control as well, but the players' two groups are the most angry.
Khornate berserkers being what they are don't have "bypass defences" in their vocabulary. They're happy little tiger idiots whose only solution for problems is to smash into it repeatedly until something breaks
I like this dynamic because the khornate berserkers don't carry ranged weapons but are godly in melee, and the tau suck in melee but are godly at range. So there's that nice dynamic where the players can win as long as they can keep close to their foes
Primitive world sounds like a way to start a longer campaign with a feral worlder group starting with very limited gear and knowledge. I suppose then it becomes a question of how do they get space guns eventually, especially if Guardsman class doesn't mean for example Imperial Guardsmen but instead feral world fighter. If a few are eaten by dinosaurs maybe a new character from off world could be introduced to give a reason and help fill out the group.
They could also just go down the route of melee or sticking to primitive weapons. One of the reasons why I was so keen to revive my old primitive campaign was just how one of my players stuck with the bow even after gaining access to space guns. I allowed him to add his strength bonus to ranged damage (normally SB is only added to melee, but I figured this would make sense for a weapon whose draw strength is... Strength), for the damage type to lose primitive quality with monomolecular arrows, and I did see somewhere that the kroot have a krootbow whose damage is comparable to being hit by an antitank rifle whilst the interex bows regularly nailed space marine armour. There's also the partisan crossbow, which managed to kill a chaos space marine with toxic quarrels and very well-placed shots.
Bow
+Quiet
+Surprisingly dangerous
+Bypasses some force field systems
-No ability to suppress enemies
-Struggles with armour
-Can't deal with vehicles
But all in all while it's not a melta gun or a las gun it is still good in the hands of a sneaky feral worlder assassinating soft targets
Does everyone get fate points or is that specifically an Inquisition/Emperor related thing?
Fate points represent people who are marked by destiny/your god of choice for great things. So each starting character rolls their starting fate points depending on their origin. Some people are just luckier than others, and it's possible for some characters to start with no fate points whilst others start with 3 or more. So it's noted that voidborn people are a superstitious lot, and they are believed to be luckier than most... This is true.
Fate points let you do lots of things like reroll a failed roll, regain some wounds or activate faith-based miracles to represent this incredible luck/destiny, and it greatly increases the survivability and success of characters who would otherwise succumb to one lucky hit from an enemy's blade. Fate points are regained after a game session but can be permanently used to activate certain abilities, or to avoid a death, to represent someone "tempting fate"
Player characters are generally assumed to be extraordinary examples of their social strata, so most should have fate points. Rare NPCs also have fate points, so it's not uncommon for a high ranking Imperial Navy Admiral to have a fate point or a traitor-inquisitor to have three. Black Crusade also adds their own infamy points to signal favour of the chaos gods but it's essentially the same thing.
You get some fun interactions with fate itself, like a tzeentch demon gaining a fate point every time someone uses one nearby it to represent it literally eating their fate. For the Afriel strain troopers, I would probably give them something ridiculously overpowered like giving them unnatural bonuses x2 to every stat (doubling every degree of success they make) but then giving them no fate points, whilst their enemies get a pool of fate points to draw on - that can only be used to target the Afriel strain troopers. So a cultist could shoot at an Afriel strain trooper and miss, but thanks to their fate point can reroll and this new result hits. This could help represent that quality of being always unlucky enough to have a bad day