The Deathwatch are my second favorite organizational group in the Adeptus Astartes. It's hard for me to call them a chapter due to them being made up of multiple chapters. I just enjoy any Adeptus Astartes that is specialized at killing. This is also why the Grey knights are my favorite above all else.
I don't think we need any more info on the legion, they are bad-ass telepathic space marines who have space cancer, who wear flames, are completely silent and only show up when the fighting is extremely messy - what more do we need to know And what more IS there to know, the lore suggests that they have no command structure, that they are all equals. Perhaps they could be an Easter egg in the game...or some kind of zone boss that the evil factions must take on
They would be great protagonists for a game, just not an MMO, although they could feature in missions as support chars for Space Marines.
or have some of the devs show up in eternal crusade AS Deathwatch members when SM really needs the help
I am just throwing Ideas around You can have harlequins for the eldar a random chaos lord and retinue for chaos and I am not so sure what the orks would get... You have any ideas?