I'm a big fan of the more open advancement scheme presented in both Black Crusade and Only War (the latter being more useful). I'm also a fan of Space Marines not being completely wooden (yes, Codex Astartes, standard tactics, blah blah. They are too one dimensional). I also like Scouts and the options they can bring to the table, but don't like that only Space Wolves use them.
I'd like to present some quick and dirty rules for using the more open character advancement rules of Only War with Deathwatch.
What're You Wearing?
The first thing to decide is whether you are a power armored Space Marine or an experienced Scout. Scouts here are not fresh neoophytes, but experienced squad members and sergeants, complete with all of the Space Marine implants. They are also a little bit more open regarding skills and talents, but don't have access to every Specialty. The difference here is how they are armed and how they have trained. This is further explained in the Specialties section.
Battle-Brother
As Deathwatch Rulebook. Note that you do not gain Deathwatch Training, Forbidden Lore (Xenos), or Common Lore (Deathwatch).
Starting XP: 1500
Scout
Starting Skills: Awareness, Ciphers (Chapter Runes), Climb, Dodge, Common Lore (Adeptus Astartes, Imperium, War), Concealment, Drive (Ground Vehicles), Literacy, Navigation (Surface), Scholastic Lore (Codex Astartes), Silent Move, Speak Language (High Gothic, Low Gothic), Tactics (Choose one), Tracking.
Starting Equipment: Astartes Scout armour, Stalker pattern bolt pistol, 3 Astartes frag grenades, 3 Astartes krak grenades, Astartes combat knife, repair cement, one Chapter Trapping.
Starting XP: 2000
Specialties
Apothecary
Battle-Brother Aptitudes: Fieldcraft, Intelligence, Knowledge, Perception, Weapon Skill, Willpower.
Assault Marine
Battle-Brother Aptitudes: Agility, Fieldcraft, Finesse, Offense, Weapon Skill, 1 other (undecided) .
Scout Aptitudes: Agility, Fieldcraft, Finesse, Offense, Perception, Weapon Skill.
Scout Equipment: Astartes chainsword, Astartes shotgun.
Devastator Marine
Battle-Brother Aptitudes: Ballistic Skill, Defence, Offence, Perception, Toughness, 1 other (undecided) .
Scout Aptitudes: Agility, Ballistic Skill, Defence, Fieldcraft, Offence, Perception.
Scout Equipment: Astartes heavy bolter, something else .
Librarian
Battle-Brother Aptitudes: Intelligence, Knowledge, Perception, Psyker, Weapon Skill, Willpower.
Tactical Marine
Battle-Brother Aptitudes: Defence, Fellowship, Leadership, Offense, Perception, Willpower.
Scout Aptitudes: Agility, Fellowship, Fieldcraft, Leadership, Perception, Willpower.
Scout Equipment: As Battle-Brother.
Techmarine
Battle-Brother Aptitudes: Intelligence, Knowledge, Strength, Tech, Toughness, 1 other (undecided) .
What do you guys think? The end goal here is to explore single-Chapter games, more representative of the kill teams presented in the novels and the original 40k rules. I like the idea of facing other, non-Xenos threats, but the "letter of the law" has the Deathwatch pretty much only fighting Orks and Tyranids. I also feel that more open advancement and single-squad play actually lets the characters do other kinds of missions, rather than just purging the galaxy with your bolters all the time.