Here's what I think the Emperor would do (well some of this, the other is just what I would do)
Ok so we are assuming zero heresy right?
1.Have Roboutte and Sanguinous run their own system wide governments. For certain I think those two would be the best leaders of governments
2.Have Curze run a giant sector of prison worlds, since he so loves punishing crime (though most of those inmates would be murdered probably)
3. Send Horus into the Web Way with Fulgrim and Pertruabo(sp?) as a team. They would be succesful with Horus as the leader, Fulgrims perfectionism and The Iron Warriors mastery of breaking down defenses.(im starting to get into my own ideas here...)
4. Find a way to remove the butchers nails from Angron's head, maybe with the help of the Mechanicus and Ferrus Manus. If it doesnt work, send him off with his warriors to go get ready for the Tyranids, send Mortarion with him, he'd be down.
I cant think of what to do with the rest.....Sang could easily govern his own sector I believe. Lorgar would have to learn to listen to daddy and not worship him. But still be the best speaker of the Imperial Truth.
Vulkan, The Khan and Corax don't get to do anything as usual. (I know whine whine...I can't help it. I WANT MORE WHITE SCAR AND SALAMANDER FLUFF!)