Originally Posted by normtheunsavoury
The Tyranids, Necrons and Tau are new threats that weren't around during the great crusade and the Empire is no longer in a position of control that it had in the wake of the fall of the Eldar.
On the other hand, the Imperium prior to the Heresy enjoyed unparalleled direction and coordination under the Emperor. By contrast, the modern Imperium is torn to a million different directions, often unable to even process a challenge until it's too late.
The only real change he could make would be to take control away from the High Lords of Terra and the Inquisition, and I'm not sure they would go quietly.
That really wouldn't be an issue. There are only a small number of High Lords of Terra, of whom one of the most influential (the Fabricator-General) already is "in" by dint of the "special relationship" Mars and Terra have long enjoyed. The Admiralty and the Lord Commander Militant of the Imperial Guard haven't ever tried to seize power for themselves, so I don't see them trying to usurp the Emperor--whom they already worship. Of the Inquisitors, only the Radicals would be a cause for concern--in fact, many Inquisitors also subscribe to the Imperial Cult.
The main thing the Emperor would achieve for the Imperium is to provide a strong focus of direction again. Able to act and speak for himself, he would be able to rid the Imperium of amazing amounts of red tape. With his influence, he could very well raise the resources to launch a new series of Crusades, allowing the established forces to maintain a defensive stance as needed while he (or his agents) proactively addressed burgeoning threats.