I think as more loyalist primarchs show up in the current setting you will see Gman start a High Council just of primarchs with himself as the "first among equals" title/powers. I expect all the loyalist primarchs not expressly killed off to show back up. Also would not surprise me if some of the dead ones come back in some form or fashion.
That would probably be Guilliman's ideal for governing the Imperium, now that he knows normal humans are too inadequate and corrupt. Most of the other Primarchs would probably be cool with that too.
But again, the Lion is the exception. We don't really know how he would react, but him allowing anyone save the Emperor or Horus have a position of higher importance than himself seems anti-thesis to his character.