Has anyone taken a swing at this yet?
I figure it would easy to build; use the Forge World rules from Dark Heresy (especially relevant with the release of The lost Dataslate) with certain choices from regiment gen that fit well (Bionics, anyone?) and not allowing the play of most Support Specializations (I'd ban everything but Storm Troopers, really.)
The rest would be renaming. Regular Skitarii are referred to as Hypaspasists. Heavy weapons teams are Sagitarii. Artillery (usually Thudd guns instead of Hydras) are called Balisteria. Your tank units are Cataphractii. Officers are called Tribunes. You would also have your usual Enginseers mucking about. I wouldn't allow Praetorians (they're basically loyalist Obliterators) because they seem rather inhuman to me. It wouldn't be too far-fetched to have Titan Legions coming in to help at plot-critical moments and Explorators sweeping in to grab the good stuff a war might uncover.
Other than that, i do believe the Skitarii would have a higher standard of weaponry. I assume most guns would be Good-craftsmanship or better, with Plasma being slightly less wonderous and Bolt weapons being a bit more common. Maybe even make the Triplex your basic weapon?