Bah, you all argue over this like children of the corpse emperor, all that matters is that we will be able to bring vengeance to our true enemies. Iron within, Iron without
Your so called Iron will not save your eternal soul since beneath the gaze of the Four we are all frail children, remember to pray the gods, they are the true power
Currently, I play as Word Bearers. Ideally Thousand Sons or Iron Warriors. I'd prefer Thousand Sons over any other Chaos Legion, but it really depends on if there will be specific Legion/Chapter perks/stats/abilities associated with each.
My preferred Race for Chaos, I would choose World Eaters if they had them, sadly they are not an option yet, So with that being said, I will choose my second favorite Race, The Night Lords is who I would go with.
At the moment it will be a Word Bearer... As soon we get Cult Legions though... I'll leap at the huggable swollen fat belly that our great Papa Nurgle has!
I would want the 5th faction to be the Thousand Sons of Tzeentch --- In the grim darkness of the Far Future you must STILL FEAR THE ILLUMINATI