Agmina Sphingis: the Winged Goddess Cleopantor gave humankind the gift of her children, the Oligarchs, who command the Space Knight Orders in the conquest of the galaxy. But the Oligarchs are not free of human flaws and the power they hold might cause terrible sorrow to the just born Dominion...
I recently started reading the Horus Heresy and just can say...
Which I think would be a great idea for a campaign although I don't know how could you manage such amount of information with a galaxy wide scenario and potentially thousands of NPCs. But it sounds really really interesting.
LIBER TENEBRARUM
An expansion to play with the bad guys: aliens, renegades, dark cults, space pirates and the like.
That's the obvious one: all the armies from W40K and other Sci fi franchises could potentially have playbooks.