I think a good path forward for the Imperium is a semi-reformation of the Legiones Astartes, except modelled more along the lines of the Legions of the Western Roman Empire. The armies of the latter day Imperium Romanum were structured as a rapid response force instead of the mailed fist of conquest.
The standard Astartes Chapter of about a thousand marines serves wonderfully as “Limitanei“ Legions, the border guards/policemen of the Empire. Given that ninety percent of the galaxy’s problems can be dealt with by a Space Marine Chapter, that doesn’t need too much changing.
Here’s where things become different. “Comitatensis” legions were the mobile field armies who responded to problems the Limitanei could not. In this spirit, “Legio Astartes Comitatensis” would be modelled after the Black Templars as fleet based formations. This would include up to five or six thousand marines.
Finally, there would be the “Legio Astartes Palatina.” The Palatine Legions were effectively the personal army of the Western Emperor, and their numbers could reach up to thirty thousand as I understand. In this regard, take the first founding loyalist Space Marine Legions and bump them up to “Legio Astartes Palatina.”
The Ultramarines, the Imperial Fists, the Space Wolves, the Dark Angels, the Blood Angels, the Iron Hands, the Salamanders and the White Scars would all be allocated thirty thousand marines a piece. These can be the force of limited conquest/hammer of any other legions that go rogue.
