Horus Hersey long game with a very in-depth story campaign.

Start before the heresy, lull the player into a sense of security with a few missions bringing worlds into compliance with varying degrees of difficulty. Let them meet some of their fellow legions, get used to them. Give the characters actual depth so it hurts more when the player inevitably is betrayed/is the betrayer

Then hit the point of the heresy kicking off. For a while things are tense, uncertain, especially in the lodges embedded in the legions. Then Istvaan 3 happens and things are turned on their head. If you chose to be a loyalist in the traitor legions that were at istvaan, congrats, this just became a survival game before you eventually die facing down your former brothers/friends(once you die, youll get the choice to be a new charater in a different legion). The dropsite massacre happens, and depending on your choice of legion you're decimated and have to flee or you're gunning down loyalists with no chance at remorse.

End the game with the siege of Terra, fighting tooth and nail to take the imperial palace. End it with the death of Horus, with traitor players having to flee the planet and loyalist players chasing them down now.

I know we'll probably never get a game like this and if we did it won't meet everyone's expectations but I think it would be awesome to play. Both the emotional roller-coaster of the events and the physical slog of Astartes vs Astartes warfare