Book 4: Filler
Last year I went to the Horus Heresy Weekender and had a great time, they teased loads of releases and shared their vision of the future of this side-project. At the time I remember thinking 'they'll get ahead of themselves if they're not careful' and I think that might have come true.
The gossip from this year's weekender is that Book 4 is called conquest and features the wider campaigns by legions currently with models. This is in stark contrast to what I heard the year before which was Prospero then Calth and then Signus Prime. Whilst I don't know the reasons behind this change of direction I suspect it's because the designers can't keep up with the books. There are still units from Betrayal that don't have models (I'm pointing at you Saul Tarvitz).
Prospero requires a lot of effort design wise, space wolves, actual wolves, wolfen, thousand sons, spire guard, Scarab Occult and Primarchs just to start with! The same is not quite true with Calth as the default models already exist for the ultras, from what I can remember only the honour guard and a few special characters are mentioned specifically that would require models for them, and the Scitarii too.
I think book 4 will serve as a buffer to allow FW to catch up with the last few units that don't have models from books 1-3 and get ahead with designing characters for books 5+. With the introduction of 3d printing to their design team it is plausible that their whole design process can be sped up to meet the game designer's enthusiasm.
Whilst I suffer from NEED ALL THE MODELS RIGHT NOW syndrome, I'm happy to wait and let the model designers get it right rather than rushing through, though I'm not sure that's the case for everyone else.
I wonder if the Heresy range is becoming a victim of its own success...