I dunno about Primarchs, but I think we can definetly be assured to see the Legio Custodes get models and stats. As this was the era when they were still running around, engaged in combat operations (however limited).
mfw the first rule in the book is "Space Marines get +1 to all stats, everybody else can go suck on it)".
But more seriously, depending on whether they want to cover any of the time when the Traitor Legions were still loyal, I would guess that the first book would roughly match up to the first 3-4 novels. With the Luna Wolves, Loken, et al., and the other famous heroes that we grew to love (or hate, like Erebus).
I think the last thing people would want in a 30k game is disappointing Space Marines.
That's your opinion, and I'll thankyou kindly to remember that.
Well, as far as I am aware, the original reasoning for the "lost legions" was to allow players to make up their own founding legions. Of course the Horus Heresy books have now made it official that stuff had happened to them before the books start, so that kind of squashes that now.
Well, that is partially true, but like the whole thing with Grey Knights being made from 'the Emperor's Geneseed', it's always been one of the dark and obscure bits of 40K lore that those two Legions, and their Primarchs, did something really bad (which makes you wonder, considering the Horus Heresy). Even before the tossed noodle-incedent style references to them in the HH series.