Imagine a six year cliffhanger like this.
The golden throne has failed, the Emperor has died, the Imperium is fracturing, Chaos is growing bolder. More worlds are falling into darkness. Xenos are expanding their territories. Worlds band together into smaller realms to look after their own. Ultramar has recalled many of their successor chapters to defend and expand their realm in an effort to hold humanity together as much as they can. Cadia and other fortress worlds have annexed many nearby worlds in an effort to do the same. Other Space Marine chapters look to their own regions and sectors in an effort to hold back the darkness. Those without a realm of their own to watch over, such as the Black Templars, Imperial Fists, and Dark Angels, rush to hot spot after hot spot desperately trying to hold back the tide. Like wise the Grey Knights do the same, fighting as never before to hold back ever increasing encroachments of the daemonic. The Imperial Church is struggling to maintain it's grip. Cathedrals and holy sites are like fortresses more so now than ever before, the ranks of of the Adeptus Sororitas dwindling as over time faith in the Emperor slips further and further, time and the lack of new devotees taking their toll. And the Inquisition? Well, the Inquisition War has resumed. Some in the Inqusition still hold true to the Imperial Creed, while others make their moves and seek greater power for themselves.
Still, all is not lost...
Light pushed back the darkness before, and it can do so again...
Edit: This is my idea for 6th edition. 7th edition would be the return/rebirth of the Emperor, second coming of the Primarchs, the Great Crusade all over again.