When thinking about possible character ideas for deathwatch, I came upon the concept of a sniper. Deathwatch are covert forces, often operating behind enemy lines and engaging in assassinations, so having someone with the team who's a little more dedicated to sniping than "yeah, I did that once when I was a scout marine..." would make sense (also, guys in power armour with powerful single-shot precision weapons are
Firstly, what chapter is appropriate? High BS will be paramount, so Dark Angels and Ultramarines would be fitting as Black Templars, Storm Wardens, Blood Angels and Space Wolves all have a pronounced tendency towards melee. Of those two, the Dark Angel chapter solo ability seems a little more useful as the character likely will spend some time away from the team.
Secondly, what specialisation would fit? The two abilities of BS and AG are somewhat antithetical for deciding it - the only one with cheap AG is the assault marine, but he's pretty much useless when it comes to better shooting while the Devastator has expensive AG. The Devastator is further focused on heavy weapons, though presumably, one could talk to ones GM to let the Immovable Warrior apply to accurate basic weapons. His ranged weapon talents are pretty good already. The third option would obviously be to go for tactical - when you don't know what else a character should be, go for Tac. As the second weapon with Accurate is a boltgun, bolter mastery sounds fine too.
Thirdly, after mulling the whole thing over a few times... what would you say are the odds we'll be seeing a Deathwatch Scout career any time soon? It really seems there's no stealthy character right now. The only one with cheap agility also gets the rather stealth-defeating feature of the humungous Jump Pack.