Evaluate your kill-team's chapters.
The kill-team is assigned to a world under tyrranid assault to act as advisors to the two companies of astartes and massed regiments of guard present.
The plan is a tried and true special. Engage the tyrannid swarm in a protracted land battle and then, when determined by the Ordo Xenos inquisitors attached to the effort, exterminatus will occur.
1) A particular regiment of the imperial guard is woefully inadequate. It is a penal levy from an imperial world and is filled with goldbricks and 4F specimens packed off by the planetary governor as criminal and heretical undesirables. Not only is there a heretical cult of "democracy" in the ranks, but also the regiment has been assigned to a key frontline position due to the expected casualties. Only the kill-team seems to understand that this position must be actually held rather than simply have the guard die in their boots. How does the kill-team handle solve the tactical problem. Do they purge the ransk themselves? Whip the guard into shape? Draft some commisars from other regiments? Check and see if the Ordo Xenos present have any buddies in the Ordo Hereticus used to dealing with this crazy "democracy" cult?
2) The two companies of astartes assigned to the world are section 8. Whatever will gall the kill-team the most, these guys are it. My personal preference, given the time setting of the core book and the recent publication of Imperial Armor 9 is to make it two companies of Astral Claws one of which contains at least a squad or two of Tiger Claws. Why won't our brother astartes let our apothecary over there? Why won't their chaplains talk to the kill-team? These guys are great fun at the pre-battle chant'n'keg and ripped through those hormagaunts, but when the shootings over they talk all crazy and laugh at us. Also, show the kill-team squads of all the same chapter using chapter squad abilities together and really working as a team. Make them realize that while they get along together, they miss their brothers back at home. (for an alternate version, make the two companies from the same chapter as one battle brother and see if he sticks with the kill-team or drifts away from them.)
3) During the orderly evacuation of the world, two strongholds of nobles and their retainers refuse to leave. One is a full-blown genestealer cult. Time to break out the flamers and go to town. The other just doesn't want to leave. They love their homeland and refuse to abandon its defense. Too bad that their industrial fief has been officially moved to another planet by order of the administratum and the imperium doesn't care about your love for your homeland. We care about you cranking out industrial parts over on this other planet and paying your tithe. So get gone already. Could it really be that clear cut? Could it they be hiding some awful xenos or chaos artifact that cannot be moved? The second noble house knows the lay of the land. With their assistance defenses in their area could be considerably more effective. Does the kill-team buck the administratum to their face? Can the inquisition end-run the beauracracy? Would anyone actually notice? It's not like the administratum can actually process all their orders. (an option for a totally amazing epilogue for a character killed during the campaign could be 500 years in the future after he has been killed, down in some stinking industrial hole on holy terra, some admistratum drone processes a piece of aged scrip, and generates another piece of scrip issuing the centuries dead kill-team leader a demerit.)
4) After a particularly vicious assault in which one of the bravest commisars present is killed an Ogryn mutiny occurs. That commisar was their favorite. They hate the weather here, the rations stink, they don't have enough ammo and their favorite commander just got eaten by a carnifex. None of the guard colnels want to go near that Ogryn encampment and get torn apart. Maybe the crazed abhumans will listen to the mighty astartes...
5) The Ecclesiarchy appear at a planning meeting and point to an extended hill off on a flank. "That's the cathedral of saint frank." they say. "The Tyrannids cannot be allowed to touch it during this the holy month of his feast. We can remove his artefacts and deconsecrate the area when the feast is ended." This creates a dangerous salient. Maybe a deathwatch kill-team can provide the oomph to acctually hold out in such a precarious position.
6) There is an outbreak of "Tranch Foot." A regiment is mutating at a terrible rate. Maybe a deathwatch apothecary can find a cause. Maybe a deathwatch librarian will have the perspicacity to figure out if there's a warp problem present or an undetected witch in the ranks.
7) One of the astartes assault veteran sergeants present on the world has finally gone loco. After a bad dust-up with a pack of lictors, he lost his squad and his mind. He's screaming about blood for the blood god and skulls for the skull throne. Strangest thing is that within hours twelve street gangs from the nearby hive had come out of the woodwork with their knives and stubbers painted up in red and howling along with him. Was this planned by some daemonic force from the beginning? The general in charge doesn't want this to be cleaned up by that marine's brothers, by the emperor, he wants some professionals to put this down. Call in the deathwatch. But what happens when the marine's buddies from the first company decide he's really their problem...
8) Whenever things are bad. Whenever the chips are down. Guess who shows up to throw a wrench in the works? The frakkin' Eldar. A bunch of lithe freaks in flashy armor with all kinds of silly tiny weapons show up. Worst of all, there's an inquisitor with them and we have to listen to him! He's even got all his paperwork in order. Who gets to escort this clearly suicidal inquisitor and his little band of harlequins off into the tyrannid infested forest on some unkown mission? The deathwatch. That's who.
9) Remember that temple of saint Frank we had to defend? Well, it turns out the ecclesiarchy forgot a couple of relics when they packed it up. Nothing important to the locals you know, just to some cannonness off on crusade. Well, on crusade nearby. Well, on crusade here now actually. Another hot potato nobody wants to touch has arrived. Guess who gets to cut back through the lines with a group of mad abbesses setting the pace? Send that kill-team that already knows the lay of the land.
10) They hit the ammo dump. The characters look around in shock as they pick up themselves up out of the pieces of the nearby guardsmen. Armor dinged up, low on gear. In the middle of a major assault. This was a week scheduled for rest and meditation. Who knew the 'nids would burrow under that river. Is that a new type of 'nid over there? We have to live through this and we have to get to a more secure HQ. (for extra fun, throw this a four days before the exterminatus is tentatively scheduled. Then have a message go out that it's been bumped up to today.)