Put simply, it's a Horus Heresy game focusing on loyal Space Marines from the Traitor Legions. Having either escaped the Istvaan system or being embedded with another Legion or something, the PCs have rejoined with the Imperium and left their original Legions. Put together as a specialist team by one of the primarchs (I'm currently thinking either Dorn or Guilliman), the team would have to face both xenos and the servants of the Traitor Legions, undertaking special missions given directly from the surviving primarchs and Malcador the Sigilite.
|I really just want to see or play a loyal World Eater.|
Picture from here.
Unfortunately, this would be a really tough game to get started. I'd have to either extensively houserule in "Chapter" rules for the Traitor Legions, or I'd have to play a different system entirely. Savage Worlds might be doable, but it would also have to be extensively houseruled. Fate Core would work, but I've never run Fate and haven't even played it very much.
Ideally, I'd want to use the Deathwatch system, mostly because it's a fairly familiar system for 40K players and it already has most of the rules I'd need. Some of the Legions would be more difficult to create than others (particularly the Thousand Sons), but the Chapter creation rules would probably suffice for most of them. I'd have to create all new Primarch's Curses for most of them, though.
On the upside, once I do all that work to create the Legions (if I do it), I'll be able to use the rules for another game I've been pondering: an alt-history Dornian Heresy game where the opposite primarchs rebelled, led by the Arch-Heretic Rogal Dorn.