This was an idea I had for the DW scenario contest but didn't fully develop it…

The Kill-team are summoned to a briefing with an Ordo Xenos inquisitor in the Tower of Brass. When they arrive they are greeted by the Inquisitor who is in the company of the Rogue Trader Gravis Terrozant (from 'The Achilus Assault'). News has reached Terrozant that a fomer human world in the Velk'han sept (Jaya) will be the site for a ceasefire agreement between the Tau and a cell of the human resistance.  After decades of conflict and with the approach of Hive Fleet Dagon the humans & Tau on this world have agreed to a non-aggression pact for the time being. The Tau are sending a water caste envoy to secure the ceasefire with the rebel leader. 

The Inquisitor informs the KT that the primary objective will be the termination of the Water caste envoy. The KT will need to make a clandestine planetfall and infiltrate the meeting point in advance of the ceasefire agreement. The Rogue Trader Terrozant has a small retinue of human fighters as his bodyguard (possible members of the 'Winter Unending' resistance?) . These are instructed to remove themselves from the briefing room for the operational details of the mission.  Once the RT's retinue leaves the Inquisitor activates a sigil on a control panel or has an inquisitorial psyker create a ward of silence around the room.

The inquisitor explains that there is another primary objective which must be completed prior to killing the Water caste envoy. When the rebel leader meets with the envoy the KT will first execute the rebel commander to make it appear that the Tau have betrayed the rebels. The RT lifts a weapon case previously lying to one side up onto the briefing table. Opening it he reveals that it is a Tau rail rifle that has been modified for marine use.  The KT will use this weapon to terminate the rebel leader and capture the kill on a pict recorder or helmet cam. This video will then be later distributed as propaganda on the human world to show the Tau 'treachery'.  Terrozant informs the KT that the alien weapon has been blessed by DW techmarines for use by the Adeptus Astartes and that they will be granted pre-emptive absolution as it will be in service to the Emperor.

The instant the human leader is dead the KT may then execute the envoy to make his killing appear as a reprisal for the death of the rebel general. The KT will be issued with a melta charge to be used to destroy the weapon once their targets are dead. The inquisition hopes that these two deaths will escalate the conflict on this world and provide a propaganda coup to the human resistance. The Tau will be forced to draw military units away from the Greyhell front and garrison them on the human world.

The KT will be secretly transported to the world aboard a ship from Terrozant's fleet (or even his own ship as he travels back & forth from the Sept). They will make planetfall and meet with a hardline faction within the resistance who are opposed to the peace treaty. This faction will provide intel, guides & transport to the meeting point of the treaty. The rebels may also provide support at the peace treaty in the form of a horde which will arrive to cover the retreat of the KT.

The resistance contacts believe that the KT's mission will be to kill the water caste envoy only. They know nothing of their former commander also being a target of the KT. They believe that the KT's mission heralds the coming of an eventual Imperial liberation fleet in the near future (in 40k terms that may be several decades). The inquisitor stresses that the 'loyal' resistance should not be disavowed of these beliefs. The world will be liberated but the Crusade cannot spare the resources for the forseeable future.

After the two targets have been terminated the KT are to co-ordinate their efforts with the resistance. They will aid the rebels by attacking high value targets in order to compel the Tau to strengthen their military presence on this world. Initially the envoy's bodyguard retinue will be tasked with hunting down his killers.  The Tau will dispatch a strike force to the planet to reinforce this retinue as well as beefing up the garrison presence (the encounter with this strike force could provide the finale to the scenario).

A couple of spanners to throw into the works : the hardline human rebels may have been already infiltrated by tendrils of hive fleet dagon. The Hive fleet does not want a unified resistance on this world any more than the Imperium does. How will the KT react if they discover that their allies are in fact puppets of the hive fleet? What if turning on the corrupted rebels means that the KT lose their intel as to the meeting point of the treaty?

Some of the rebels may stumble upon evidence that the KT have more than just a single target at the peace treaty eg. the weapon case for the rail rifle breaks open as it is loaded onto a transport; one of the rebel guides at the meeting point approaches the KT location and discovers them in a firing position with the Tau rifle. Will the KT try to explain to the rebels that what they do is "for the greater good of Humanity" (something which the partisans will find dangerously similar to Tau propaganda) or will they silence the rebels permanently in order to maintain mission secrecy?

The reference to Enslaver Plagues in the Deathwatch Rulebook set me to wondering how to use these horrors as opponents. Since Enslavers aren’t combat powerhouses, the bulk of the challenge will be fighting through lots of enslaved humans. The tricky part is making it ‘feel’ different from just putting down a conventional rebellion. Here’s what I came up with:

When returning from a mission, the Killteam has to transfer from one vessel (an Imperial Navy cruiser) to another (a small escort ship), as the cruiser is urgently needed at the battlefront. The ship the Killteam is on arrives at the rendezvous point late due to some warp turbulence, but the escort that is to take them back to the Watch Fortress is no-where to be found. They scan the area and get a hit at extreme auspex range: the ship they are to rendezvous with is on course toward the nearest inhabited planet- a xeno-held world- at maximum plasma-drive speed. The stray vessel refuses to respond to hails from the cruiser, and the Astropath is unable to make contact with his counterpart on the escort vessel, saying he can only sense a terrible, non-human presence. Assuming that the ship has either been seized by aliens or willfully turned traitor, the cruiser mounts a boarding action- naturally deferring to the Killteam to take the lead.

Once on board the escort ship, the Killteam and their naval allies have to fight through hundreds of glassy-eyed crewmen, at first poorly armed but eventually equipped with gear that can actually threaten a power-armoured Astartes (such as welding equipment and Sentinel walkers with power lifters). There are occasional signs that someone on board ship is helping the Killteam, by doing things like closing blast doors to seal off hordes of enslaved crewmen. They discover that there are some loyalist techpriests barricaded in the Enginerium (they are the ones who sabotaged the ship’s warp drive, limiting it to slower-than-light travel). They won’t allow one of the sacred machines of the Omnisiah to fall into xeno hands, so they have rigged the ship’s reactors to overload in a matter of hours. The Killteam has that long to root out the Enslavers who have taken over the ship and turned its Navigator and Astropath into living warp gates.

I've been cutting the hell out of my campaign, so this isn't going to end up in it sadly, but I thought it was a good enough idea to share it with you.  KT is given a mission - something fairly standard involving escorting an Inquisitor (my plan was to use Quist but that's for campaign reasons).  They set off, you have stuff on the ship, you have them arrive - and find out that whatever reason they were there for is gone.  Xenos, cults, artefacts - they were destroyed/left/whatever.  So the KT and the pissed off Inquisitor pile back onto the ship, set off back for Erioch.  On the way however, the ship is attacked, or its drive malfunctions, or both, and the KT end up scattered in the wreckage with the Inquisitor on a planet somewhere.  

Some investigation reveals that this world is non Imperial - it hasn't yet been touched by the Crusade, or the xenos or chaos (that the KT can see).  I was planning for early 20th century levels of tech, but you could have anything, any setting (save primitive jungle, it's overdone).  They clearly believe in something that's the God-Emperor, even having a statue of him.  They obviously know nothing of the Imperium, so to them the Inquisitor is just someone bizarrely shouty, and the KT are just awesome warriors.  From here you can run whatever plot with them you like - maybe there's some ancient communications device on the planet, its location shrouded in rumour that would allow the KT to tell the DW where they are, maybe they can rig up a communicator but it's going to take two plus months for them to be retrieved.  Have ammo issues, have power issues - have the KT having to rely on the planet's tech.  Let there be interfering xenos - I was going to have Dark Eldar, but you could have vanilla eldar or tau or chaos or necrons.  

Also have the Inquisitor be a dick to everyone.  The KT should be torn between their job - protecting the Inquisitor - and trying to prevent them from getting them all killed by pissing off the natives.  

I was also going to have a trapped demon prince, a C'tan pyramid, a recurring Obliterator villain and a crushing maiden, but that's just my flavour.  

Here's something I came up with to take a Killteam out of their comfort zone:

After completing a mission, the Killteam is 'cooling their heels' in an Imperial Guard camp near a warzone, waiting for their transport ship to arrive and take them back to their Watch Fortress. While they are waiting, a courier arrives with an urgent message from the local General, and, as a courtesy to the scions of the Emperor, the local Colonel invites the Killteam to attend the briefing. It seems that a high-value target has been identified in the area: a Tau Ethereal (and his bodyguards) who is hiding out in a 'no man's land' area of the battlefield, apparently conducting some kind of intelligence operation. The Colonel states that he simply doesn't have the manpower to dedicate to hunting down a possible 'bogey' without jepordizing the line he is charged with holding. If your players are anything like mine, they will gladly volunteer to go on an impromptu 'search and destroy' mission…

So far so good. But when the smoke clears and the Killteam is searching the Ethereal's corpse, they find a series of communiques between the Ethereal and an Inquisitor, arrainging for the Ethereal to defect, bringing vital information about the Tau's troop placements with him. When they check with the General who sent the info about the 'target', he says he sent no such message! The Killteam has been duped, used to assassinate the Ethereal by a traitor loyal to the Tau somewhere within the Imperial forces. The Inquisitor will be arriving in a few days (on the same ship that is to transport the Killteam home); if they want to avenge their dishonour, they will need to root out the traitor- potentially an even higher value target than the Ethereal himself- before the ship arrives…

Rumor has spread that an ancient, pre-Imperial Star Map has been located in Vanity's underhive, a planet already in a warzone. Apparently the map indicates the location of an Eldar Exodite world within Imperial space in the Jericho reach. Some puritans already want to destroy the map as heresy but the Lords of Ordo Xeno's believe the map could be true and may confirm their suspicions that the Eldar have technology that mask an entire solar system from perception in the warp.

Proof of this concept could affect strategy across the entire Imperium. Finding this Exodite world, which may even be a Maiden world, would not give the Imperium access to the resources of the Eldar system and enable them to deny it's use to the multitude of Corsairs which constantly plague the Imperial merchant vessels.

However the Biel-Tan craftworld seem to have been made aware of the map and are trying to destroy it themselves to protect the world. The normaly obtuse Eldar keep their thoughts to themselves but in a rare frank exchange with a Farseer the Ordo Xeno's learned that, to this particular group of Eldar at least the Imperium is their archenemy, due in some part to some perceived 'genocide' during the great crusade and afterwards.

Inquisitor Rathamus was sent to locate the map, but encountered some of the Eldar forces. His entire retinue has been destoyed and he is wounded. Infiltrate the lower levels of the hive, rescue the Inquisitor and defend him as he tries to obtain the map. Be alert the Eldar cannot move freely in the hive and they will be using their foul 'magics' to appear human or to not appear at all. In addition, the Chaos forces that are probing the planets defences might move against you once they spot you in the area; there is a an ancient secret human society dedicated defending the Eldar due to pre-Imperial co-operation with the race; there are parts of the Adeptus Terra and even the Inquisition that may not want you to find the map, either due it's apparently heretical origin or just their unwillingness to open up a fourth front against the Eldar; and final watch your footing in the underhive it's dangerous place for even Astartes, who knows what might be lurking down there.

Here's one mission idea that I want to have my Deathwatch Killteam experience as a one-shot scenario.

Donkey Kong.

An extremely large and heavily mutated ape-like creature is terrorizing a Hive city on a Forge world key to the Imperium's crusade in this sector. Just so happens that an Ordo Xenos Inquisitor is at this Hive and requests that the Kill-Team help rescue one of his cadre, a Sister of Battle of no small repute who happens to be carrying and item of importance (left up to the GM as a hook for future campaign).

As the team closes in one creature, it makes its way up into the upper spires of the hab, wreaking destruction and causing chaos wherever it goes, until it makes its way to the uppermost Spire of the Hive.

What you do with the "ape" is entirely up to you. It could be a creature bio-engineered by any particular race that delves in such matters, even the Magos Biologis. It could be some warp-spawned abomination let loose by cultists. I'm just throwing out ideas here for fun.

Drop Assault: While engaged in a fleet battle with chaos ships in low orbit over a war torn planet, the players are in the armory aboard a small Imperial Navy ship, equipping for a planetary assault on a fortress. However, while they still have only limited supplies equipped, the hull of the ship is shattered, and the players are blown out, hurdling down toward the planet. They are fortunate enough to fall onto another capital ship. Unfortunately, it's a chaos ship. Having taken a fair amount of fall damage and being low on supplies, they need to get inside and take out the chaos ship, as the space battle rages on above. After they take over the ship and crash land it into the fortress on the planet's surface, it's time to begin the assault.

Joint Ops: Have the squad prepare to Air Assault an objective with gravchutes out of a Storm Raven Gunship. However, one marine stays behind to act as the gunner in the Storm Raven. The squad must advance on the objective as the gunner helps them take out targets from above, providing some extra firepower. Once they reach the objective, however, the Storm Raven is shot down. The squad has to go rescue the downed crew and teammate from the approaching defenders.

Armored Spearhead: Assign the Killteam a Predator tank, and have each member fulfill a different role: driver, turret gunner, pintle gunner, commander, etc. Their mission is to break through to a group of inquisitorial storm troopers and their inquisitor who are surrounded by your choice of enemy: Traitor Guard, Orks, Nids, etc. After they have reached them, bring the Inquisitor back to base.