I think that it may not be like a single line at all. Instead, it may be similiar to Star Wars Battlefield, or Counter Strike in that you launch right in, and when you die, you come back in as re-inforcements, but over a series of campaigns.
If anyone ever played Return To Castle Wolfenstein: Enemy Territory, you had a re-spawn timer, but you could choose what kind of marine class you were.
Say you could have a basic marine, flamer equipped, or heavy weapons w/ a random heavy weapon.
Then say, that as you kill, you gain xp points in certain areas of being a marine. As you advance through a cyclical say, 6 maps, you retain ur xp points and get better stuff.
Quick turn around, non-stop action, and all set in the 40K universe. Only problem I see is, it'll be hard to let people play the particular race they want. You'd have to have campaigns amongst certain groups that reflect major fluff battles.
I don't know what they'll come up with, but you can be sure it'll be interesting!