I'd love a series about the Age of Apostasy/Reign of Blood, ending with the founding of the Ordo Heresticus perhaps?
Hell why not start it with the Cursed Founding (21st Founding) which occurred only decades before?
yeah, AoA is just begging to be novelised
I would say Goge Vandire came just as close as Horus to destroying the Imperium, it could even be argued that he got closer (after all he managed to become the Imperium's sole dictator)
I think Vandire would make for a fascinating character
I think the fact that he wasn't a chaos-charged superhuman primarch is a good thing, it would be interesting to see how a regular human being (albeit an ambitious, devious, and cunning one) brought the Imperium to its knees, I think that the fact he had nothing to do with Chaos (apparently) is also a plus...it shows that many human threats to the Imperium are not chaos-fueled
for instance the Imperium would probably find the ideas of democracy, progress, individual freedom, pursuit of happiness, and tolerance to be very threatening...the Inquisition would label those who espoused such ideas as "Heretics" and maybe trump up charges that they're in league with Chaos (Tzeentch and Slaanesh maybe)
I think 40K actually has potential for a great deal of depth