Board Thread:Warhammer 40k General Discussion/@comment-25618639-20141119150740/@comment-2093048-20141205200132

Gulliman set up the High Lords in the first place. I'm sure he'll figure out a way to make his return seamless. And I doubt he'll want to seize full (or possibly any at all, even) political power either because... well, again, he's the one that set up the High Lords and divided his power in the first place.

The Emperor's original vision was that normal humans would rule the Imperium, not the primarchs, which is why the Emperor set up the mortal council that Guilliman eventually refined into the High Lords. The only truly major deviation the Imperium of today has from the Imperium of Gulliman's final days is the Ecclesiarchy. Of which I doubt Gulliman would challenge because Gulliman actually felt guilty about burning down the Word Bearer's Monarchia holy city and only did it because Emps ordered him to. Without Emps to order him to burn down the Ecclesiarchy, he likely will just let it lie just like he would have done with Monarchia.