Deathwatch is one of my favorite armies in terms of fluff and background and thematic elements (I really like the idea of Astartes “Special Forces” drawn from all different Chapters and equipped with the best equipment, but always fighting outnumbered and out-gunned), so ideas for how to make this codex both more fluffy and more competitive are always coming to mind for me.
One thought I had was to allow each Watch Sergeant in a Veteran squad (or Watch Captain as an IC) be able to “purchase” a Veteran skill that would apply to him and his unit and would reflect him teaching the squad methods of warfare that he is especially proficient in, based on what Chapter he originally came from. For simplicity’s sake, I thought it would be best to just make one skill per the original 9 Loyalist Chapters (after all, they are the primogenitors of all the subsequent chapters, so they could be used to represent Watch Sergeants from other Chapters as well) and to limit each “veteran skill” upgrade to one per army (i.e. so you could only have one Space Wolf background Watch Sergeant and one Blood Angels background Watch Sergeant, etc. etc. per army).
Here are my ideas for this:
 “Tactical Acumen” — Ultramarine Veteran Skill (15 points) — Once per game, starting at the beginning of the friendly Movement Phase, this model and all members of his squad with the Deathwatch faction may re-roll all to Hit rolls until the beginning of the next Friendly Turn.
 “Bane of Tanks” — Imperial Fists Veteran Skill (15 points) — This model gains Tank Hunter USR.
 “Advanced Bionics” — Iron Hands Veteran Skill (5 points) — This model gains FNP(5+).
 “Firestorm Tactics” — Salamander Veteran Skill (10 points) — This model and all members of his unit with the Deathwatch faction gain Shred for their Hand Flamers, Flamers, Heavy Flamers, and Heavy Flamer portions of their Infernus Heavy Bolters.
 “Cunning Combatant” — White Scars Veteran Skill (15 points) — This model gains Hit and Run USR.
 “Shadow Tactics” — Raven Guard Veteran Skill (5 points) — This model gains Shrouded USR for the first game turn, provided it did not start inside a vehicle.
 “Battle Wrath” — Space Wolf Veteran Skill (5 points) — This model and all Deathwatch faction models in his unit gain Counterattack USR.
 “Storm of Fury” — Blood Angel Veteran Skill (10 points) — This model and all Deathwatch faction models in his unit gain Furious Charge USR.
 “Unrelenting Fortitude” — Dark Angel Veteran Skill (5 points) — This model and all Deathwatch faction models in his unit can fire Overwatch at BS2.
As you can see, these skills basically mirror at least a portion of the “chapter tactics” of the original 9 Loyalist Chapters and, I think, would also be competitive choices customizing each DW Veteran squad a little further. The Watch Captain could add even more flexibility by taking one of these skills and attaching to a squad that has a different skill via its Watch Sergeant… so, for example, you could have a close combat focused squad that has both Furious Charge and Hit and Run or, alternately, you could have a squad with both Tank Hunters and the ability to re-roll all to Hits once per game.