I don't see any of the four specific legions that are dedicated to one god being put into the game as its own thing. I think that they will add the Alpha Legion because they are undivided. I do believe that once the game starts you will get a choice of picking your favorite god or going undivided. Once you pick the god you will have to work your way through the Tech Tree for you specific legion choose. Ex: I pick Khorne as my god and I work my way through the Tech Tree, at some point I will get to become a Khornite Berserker and the same with the other gods.
Don't get me wrong, I love the Thousand Sons but there's this whole problem of them all being powerful sorcerers or mindless automatons. I mean, if they can find a way to do that, I'm all for it but until then, I'm voting Alpha Legion.
That's how I see them doing it too, Morentz. I hope that they don't make the 4 chaos gods each have powers that are common to all CSM's - like "Tzeentch Bolt" being an attack and "Sweep of the Blood God" being a melee attack - that would be cheesy - but rather each God having a tree, and undivided gaining access to multiple trees of different gods or choosing a single gods tree if you devote yourself - something like that.
Are you all Thousand Sons lovers suppose to play sorcerers? No way that will happen, I say. And how do you suppose to play rubric marines? The rubric marines have no their own will, only spirit identity, but I hardly imagine hundreds of human players blindly following chat commands just to roleplay rubric marine... That's dumb overall.
Out of those choices I'd say Alpha legion as the warbands already in are undivided. Otherwise there would be an over represented ruinous power and marks are coming in at a later date after lauch (IIRC)
It'd probably be best to do the Legions that are dedicated to a single god as individual classes rather than a subfaction. Noise Marines, Berserkers, Plague Marines and Sorcerers.