See, I doubt they'll come out with Primarchs for game balance reasons. They are simply to powerful for any scale smaller than Epic. I mean, it sounds cool, but it just doesn't fit the fluff at all to have Leman Russ or the Deamon-thing that is Fulgrim die to a bunch of Lascannon shots from a guline gaurd army. Also, how do you come up with rules for a Warp-Forged Super-being which has been personally leading an entire legion of Space Marines across the galaxy in a crusade of conquest for centuries? Fearless, T10, I10, S10, W10, A10, BS 8, WS 8? It's just not practical from a rules standpoint.
This is not to say it wouldn't be cool to do a display model of them, just for the fun of it.