With the Forgeworld Horus Heresy books fast approaching, I'm trying do decide on a Legion to collect. Although my Thousand Sons are in the works, they aren't likely to be included in the first book, and I want to be able to use the Pre-Heresy rule set straight away. I can always include my Thousand Sons as Grey Knight allies if I want to field them.
Now logic would dictate that they cover Istvann III and the Dropsite Massacres in two books rather than one, otherwise they've got to cover 11 of the Legions in one book.
With this in mind, the most likely
candidates for inclusion off the bat for the traitors are Luna
Wolves/Sons of Horus, Emperors Children, Death Guard and World Eaters,
as the Legions present at the intial betrayal.
Of these, I don't like the Death Guard (they just don't inspire me), leaving me with a choice of Son of Horus, World Eaters or Emperors Children. It's very tempting to build a small force of each of them, but that would very much depend on how much time and money I have.
Of course if they cover the Dropsite Massacres in the first book, it's going to be Night Lords, no question, but they can wait until I know they have printed rules.
To be honest I can see myself having armies for most of the traitor legions, complete with Primarchs by the time this is done, but right now I'll settle for one. The question that's vexing me right now though is, which one?