Looking nice!
A couple of suggestions:
Daemons
A lot of the Siege of Terra background has daemons taking a fairly major role – bloodthirsters rampaging around, hordes of monsters embattled at the Gate etc., so I'd really like to see more daemon thingies available to the Renegade legions – and I'd imagine there're a lot of old-skool cult players who have some 'proper' cult daemons in mothballs.
Of course, I don't want to steal the dedicated Daemon players' schtick, so I'd kinda like to see these limited to the cult legions at this point – perhaps a special
Corrupt rule given to the Legion Tactics that allows daemon packs – but only of the relevant god; and with Word Bearers (and possible Sons of Horus?) retaining the ability to mix-and-match, summoning daemons from all their filthy gods

.
If Alphas, Night Lords and Iron Warriors desperately want daemonic allies, they've still got lesser daemons, representing their lesser devotion.
I also think that Possessed marines are a cool visual link to how debased and monstrous these twisted Astartes have become (and we have the special Red Angel possessed already). Perhaps these could also appear in the Cult legions and Sons of Horus/Word Bearers?
If you're concerned about balance, you could create special character(s) for the different legions that allows access to Possessed and relevant daemons – but at the cost of limiting some other items. I'm not entirely sure what would be limited (p'raps vehicles/transports to offset the increased mobility, or Vanguard and Sternguard to represent the veterans giving themselves over to Daemons?), but then, I'm not writing the packs!
+++
Daemon Packs
I'd also echo Lucifer216's point about the 'fleeting aid' available to the Word Bearers. While I think it's a nice limiting factor, I'd mute it somewhat to the following:
1) Units taken as daemonic allies never count as scoring (this prevents 'objective swamping' by monstrous creatures).
2) Alter the 'remove the unit on D6 against game turn' to 'test on Ld with current game turn modifier – take a saveable wound for every 1 you fail by'
e.g. Third turn, a pack of Bloodletters tests against their LD at -3. For each point they fail by, they take a wound and must make a save.
The reason I like this approach is that it's not so dependent on luck (so no horrid disappointments for the Word Bearers as all their daemons disappear at once

), is scaleable, and the opponent can affect it with Sisters of Silence etc, which I think would be a nice touch, and hopefully encourage their use – they were hugely outnumbered by the Big Gold Buggers last time.
On a related note, does the current 'fleeting aid' thing only apply to the Word Bearers? I'm not too
au fait with Codex: Daemons, so I'm not sure if it applies to units taken by the other legions.
Hope that's useful feedback – looking forward to seeing how the Thousand Sons appear!
