An unfounded theory presents itself to me. This may be completely innacurate though.
Schedule-wise, apparently we were meant to see the official Horus Heresy Painting Masterclass book from Forge World in May.
We got the paints but no book. Now I have to question why. My possible theory is below.
A Brand new Space Marine Codex has just come out with some new Minis (Assault marines and Devestator marines).
We have had 2 weeks of no releases from Forge World on a Friday (22nd of May the paints came out). This is not FW's usual MO, always something every week.
So my wild Theory is; Masterclass Book has plastic Horus Heresy Space a Marines in there.
GW wanted to release a 40k Space Marine Codex with all these shiny new Marines for the pure 40k players, new model box sets with the new bases etc to get their house in order before they release an Age of Darkness Space Marine Supplement (basically ensuring they keep the 40k Space Marine gamer supported and happy before creating a parallel range).
We all know Games Workshop's busy release schedule has pushed it all back which Forge World had not bargained on.
Now this could be totally unfounded wild rumour making, but it does strike me as strange the book didn't come out in concert with the paints. When speaking to the excellent staff at FW we get a teasing "watch this space" when we ask for a paint scheme which utilises the new paints.
I am fully sold that there will be a plastic range this year. I believe this by just looking at the upgrade sets that FW are making instead of the beautiful complete kits they used to make.
We are also seeing a lot of "start your Legion now" banners on FW selling the standard Legion pieces we all will be buying anyway like this;
Final confirmation nail in the coffin for me will when we see those Breacher shields, arms and guns we saw in Tempest (older square pattern type) sold without bodies.