The whole of the Gothic War, or an even earlier Black Crusade. BL/GW don't need to go into M42; with 10-12000 years of Imperial History basically unexplored, going beyond the 40k 'present' would be something to do just for the sake of doing. Besides, any ideas and themes such a progression might bring up could always be fitted to an earlyer event. The Astronomicon failing? Write up the events of a world on the fringes of the galaxy where the Emperor's light has disappeared. The Emperor dying? Just show us a system, set at almost any point after The Scouring, were such a belief has come to the fore, and the Imperial leadership is struggling to keep order and the leaders are struggling with just what the hell it means for the God-Emperor to be dead.
The past hides many treasures; and even more terrors!