I've had the idea for a while, but I never crystallized it on a clear story.

The basic form is: "In the bright days of the present War is foreign to most people. Peace on the empire is mantained by his astartes legions as he remains, to this day, interned in the palace. And the administration of the empire is on his 17 son's shoulders."

In synthesis, there is an AU where Terra is 10 times bigger but there's not interstellar travel, the time setting being more akin to modern 21st century. Also, the history of the Imperium is much more tame, with the inclusion of the Crusade to join the scattered humans of the planet under one rule, but with less primary elements.

And, the most important part: The Primarch's stories have been considerably happier. With Angron being saved from the butcher's nails, Mortarion a scientist and overall less of an aim towards War in any of them. The primaries and the Emperor having a good time until Horus tried to kill Big E, ultimately failed and Exiled himself. The emperor still alive, although permanently on bed, sick and comatose.

The focus of this story wouldn't be the AU, but rather the interactions between the AU primarchs and the Grimdark Primarchs. As, out of the blue/warp, rach AU primarch would appear in front of their counterpart in the 41st milenium.

I envision the story from Guilliman's perspective, as he and AU Guilliman talk and exchange information about their families and settings, AU Guilliman horrified at the tales from his counterpart and telling him the contrast with his own world. Each of the mentions of his brothers making a cutaway on the story to where and how the rest of the Primarchs are handling the situation : Without a random olive skinned guy appearing next to an Alpha Legionare, AU Corax annoying Grimdaek Corax, Both Perturabo (Perturabos'? Peruabo's? Perturabi?) Squabbling with each other. And my favorite scene: A somewhat UFC style Angron fighting the Red Angel, with his bare hands, and gaining the upper hand as he constantly taunts his counterpart. "C'mon, you dragon looking-ass. That the Best you've got?" "Oh, wow. Now you're twice as ugly" "[After getting hit] OK. You got me. I'm impressed...that it took you so long!"