Me and my Brothers i Arms took up this question with our GM.

We suggested a number of plots and ideas to develop roleplaying around, for specific missions, side-plots and just for the fun of it. Please add, adjust or suggest other ideas.

BACKGROUND

"Each Space Marine may serve a discrete amount of time, or for the
duration of a mission, which may be a number of years. Once the
mission is complete, the Space Marine is allowed to return to his
chapter, under an oath of silence, as their duty has been fulfilled.

The orders of the Deathwatch are not merely the cleansing of xenos
cultures. They also include the recovery and study of alien devices
and artifacts. Sometimes it is necessary to use a weapon against the
enemy who created it, although this is not taken lightly. The
Deathwatch are constantly vigilant for sabotage, or to advise if it is
truly safe to use a weapon of xenos origin. The Adeptus Mechanicus are
always on the lookout for alien technology; the C'tan Phase Sword,
used by the Callidus assassin was recovered from a Necron tomb world
and successfully integrated into the arsenal of the Imperium."

HERE IS A SHORT LIST OF SUGGESTIONS (based on that I am playing a Dark Angel Techmarine):

- Interaction (conflicts, friends, competition, conspiracies...) with
the Forge(s). Both at the Dark Angels Chapter Monastery and at the Deathwatch
Fortress.

- Test of loyalty to the Dark Angels Chapter? Search and recovery of an alien artifact and/or a lost relic from the Dark Angels Chapter, maybe a 'lost' comrade or from one of 'The Fallen' that have stolen a unique item from the Dark Angels Chapter!
''Not all of the Dark Angels' damned brethren have succumbed to the
power of Chaos to the same degree. Some of the Fallen have embraced
the ways of the Dark Gods totally, becoming true Chaos Space Marines.
These Fallen do not belong to a warband or Legion of their own, like
Angron's World Eaters. Instead they are dispersed throughout space and
time as either isolated individuals or in small bands.''

- Test of loyalty to the Adeptus Mechanicus? Maybe a contact/friend from the Forge World and/or Adeptus Mechanicus asks for services or surveillance of one
group, organisation or individuals?

- Preparation and collection of materials for cybernetics, implants and Servitors.

- Preparation and contruction of the Astartes Artificer Armour (Rank 4: Techmarine + Hero).

- Investigation of one or more suspicious 'Black Shields' within the Deathwatch team/fortress?
http://wh40k.lexicanum.com/wiki/Deathwatch_Black_Shield

- Other Dark Angels within the Deathwatch organisation? Are they inluential, involved in a power struggle, friendly or just
troublemakers - or one of the Black Shields, and why was he banished/excluded from the Dark Angels Chapter?

Great list, some other possibilities:

-One of the DW Librarians, contacts you telepathically right while you're in the middle of processing orders regarding a fallen/ dealing with a fallen/etc. He might have picked up on your thoughts...what do you do about it?

-A watch captain is a deathwing vet, and keeps sending your squad against a cult/traitor guard/ etc. is it Fallen related because he keeps tasking you to lead or just favoring a member of his former chapter?

-An Inquistor knows about your 'unusual behavior' on certain missions, and chooses to bring his retinue with you on a few missions. One of those missions, you or the Inquisitor comes upon a fallen. If its you, he does an 'angels of darkness' confession and the Inquisitor, hears pieces of it. What will/can you do?

Brother Zahariel said:

Thanks.

I don't understad this part!?

As a big DA fan I've read all the Black Library Novels one of which being "Angels of Darkness" where long story shot a Fallen tells an Unforgiven his version of the fall of caliban,