The only Primarchs we've actually had any real characterisation from are Horus, Lorgar, Fulgrim, Jonson and Magnus.
Russ has popped up in a few places, but didn't get anywhere near as much coverage in Prospero Burns as I'd hoped.
Corax was a badass in First Heretic, despite pussying out and not finishing Lorgar off, but the descriptions were conflicting with that of Raven's Flight, like where was his power whip? I wonder if we'll get one about his post-Istvaan experience. That seems to be the key area for the Raven Guard.
Mortarion gets some coverage as well, but not nearly enough in Flight of the Eisenstein.
Dorn, Guilliman, Sanguinius, Vulkan, Ferrus, Kurze, Peturabo and Angron have all appeared, but only as cameos.
Khan's the only one that hasn't even appeared yet (correct me if I'm wrong).
The problem is that most of the latter group don't have a significant enough part in the Heresy, apart from the attack on Terra itself, except for those at the Istvaan massacre.
I think, and certainly hope, that we'll get more of Vulkan and Corax, as Nick Kyme's Salamander books have proved popular, so hopefully we'll see a HH Salamanders book. If they do publish a Raven Guard book, it'll probably be the events leading up to Istvaan, the Massacre, and then the fallout afterwards when he goes back to Deliverance.
Dorn will get fleshed out as they approach the battle for Terra - I imagine they'll start pumping those out after Age of Darkness, as will the rest of those involved in the battle, so that includes Sanguinius, Khan and the other traitor Legions.
I sincerely hope Aaron D-B does the Night Lords book, as they're hardly anything on their founding times - I think Kurze is one of the most interesting of the Primarchs.
I can't see how Guilliman will get any stage time, seeing as the Ultramarines were sitting on the bench for all of the heresy - maybe post-heresy when he has to split the Legions up?
Angron and Peturabo will probably get books of their own - all the traitors probably will get books detailing why they fell to chaos, though someone would have to do a good job characterising Angron...

I just want more from Graham McNeil and Aaron D-B - I've gone off Dan Abnett after Prospero Burns - that book was a major letdown in terms of background.