Nothing stops you from doing a mish mash, here most of the legwork ought to come from the background reasining.Are there any rules for Only War where you could have something akin to deathwatch where guardsmen from multiple different regiments form killteams and taskforces to deal with certain missions?
Yup, one to be accurate.No. Have you?
The return of a primarch would be seen as a miracle regardless of whoever came back, that much is certain.Question: What if Vulkan and not Gulliman came back after the 13th Black Crusade? How would the Imperium change because of this?
That'd be true, Gulliman is a statesman, after all. But Vulkan would probably be the next-best thing, as he commanded the Post-Heresy Imperium before in the War of the Beast.The return of a primarch would be seen as a miracle regardless of whoever came back, that much is certain.
That said, I believe Vulkan wouldn't be nearly as good as lord commander as Guilliman considering his mindset which, I believe, would lead to conflict between him and other branches of the Imperium such as the Inquisition.
Hell, Guilliman may very well be the only primarch with enough common sense to avoid that.