The more I think about it, the more it seems to me that, in order to maximize their combat potential on the tabletop, the Deathwatch need some form of cheap, “cannon fodder” allies who can take objectives, absorb enemy firepower, tie up units in close combat, and just generally provide balance the high-damage-output-but-expensive-and-relatively-fragile Deathwatch units.
Looking through all the potential Battle Brother allies (i.e. Imperium codexes), of course Astra Militarum jumps out immediately, with their access to cheap units, long-range firepower, and lots of supporting units like Wyverns, Hellhounds, etc. to help even out the numerical/durability disparity that most Deathwatch armies will face.
Outside of Astra Militarum, however, I think one of the best options for cheap, “cannon-fodder” allies for the Deathwatch is the Wolfkin Formation from the SW Codex. Costing 8 points a model (or almost 1/3 the price of a DW Veteran), a Fenrisian Wolf is basically a Space Marine without any BS/shooting options and with only Ld5 and an Orky “T-shirt save” of 6+. However, each Wolf has 2A base and, if the Wolfkin form an “Alpha Pack” of 20 or more models, then each Fenrisian Wolf gets 3A base (or 4A on the charge). On top of this, since Fenrisian Wolves are Beasts, they move 12″ a turn, are not slowed by difficult terrain, and have Fleet, making them great for getting across the board quickly and seizing objectives/tying up enemy forces in CC early in the game. Bottom line, they provide a cheap, expendable, but still fast and decently dangerous (in CC) ally to take attention away from the fragile and expensive Deathwatch forces.
Despite these advantages as allies to the DW, Fenrisian Wolves from a Wolfkin formation still have abominable leadership and very poor armor, so how can a Deathwatch commander mitigate that when employing them as allies to his main force? Well, you can attach a Watch Captain with AA, Jump Pack, Storm Shield, TH, and the “Dominus Aegis” relic and attach him to your allied Wolfkin “Alpha Pack” of 20+ models. Not only does the Watch Captain give the unit Ld10 and ATSKNF (mitigating their Leadership issues almost entirely), but he can keep up with them using his Jump Pack, tank wounds with his 2+/3++ save, and bring some S8 AP2 Thunder Hammer “heat” to supplement their massed S4 attacks. Perhaps most importantly, with the Dominus Aegis, he gives the entire unit a 4++ on any turn that it did not move in the previous Movement phase… while this will likely not help early on, as the Watch Captain/Wolfkin would generally be moving up field in the early game to take objectives and apply pressure (or at least absorb pressure away from the DW units), once they get locked in combat following the turn they charged, they by necessity CANNOT move in the Movement phase prior to those follow-on rounds of combat, thus bestowing on the Fenrisian Wolves a 4++, which obviously takes the durability to an entirely different level and, combined with Ld10 from the Watch Captain, allows them to be a highly efficient “tar pit” unit for key enemy models and squads that you do not want attacking the rest of the DW army (on a side note, the Wolfkin formation also has Monster Hunter, which would be conveyed to the TH-wielding Watch Captain attached to them, making him much stronger against enemy MCs/GMCs).
The key this whole concept, I think, is to use the Watch Captain/Alpha Pack battle group to control the board and direct attention away from you Kill Teams, Veterans, Vanguard squads, etc. etc. The Wolkin obviously are not the most “killy” unit, but if they can absorb enemy attention and combat power efficiently, that frees up the DW units, which do have lots of combat power, especially in shooting, to be able to apply their firepower and close combat potential to maximum advantage while minimizing the casualties they take in return. Deathwatch units could either Drop Pod in as the Fenrisian Wolves advance turn 1, putting lots of pressure on your opponent right away, or they could “follow in trace” of the Fenrisian Wolves and use them as screening element as they move into position and shoot (i.e. whether footslogging, riding in transports, or even DSing in later turns using the special rules of the Black Spear Strike Force). You could also take an Allied Ordos Xenos Inquisitor (quite fluffy, right 😉 ) with Rad Grenades, the “Liber Hereticus” relic, and Servo Skulls and attach him to the Wolfkin/Watch Captain battle Group. The Liber Hereticus allows the entire unit to Scout move and, once a game, gain Hatred, the Rad grenades make the whole unit much better at close combat on the charge, and the Inquisitor adds Stubborn to make the unit even less likely to fail its Morale checks. Finally, the Servo-Skulls allows for more accurate DS by friendly Deathwatch forces, further enhancing the army as a whole.
While this idea is not perfect, I think using the Wolfkin formation as an ally for a Deathwatch army could be quite effective, especially in the hands of the right commander who can get the most synergy out of these two forces working together (plus, it means that your army has a lot more bodies for the enemy to deal with, mitigating the low model count that is problem for most DW forces).