system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] Is there any possible scenario where the Imperium of Man can return to it's past glory?
gpt
No. Mostly because there was no past glory for the Imperium of Man. Even at its height, before the Horus Heresy, the Imperium controlled less worlds than present. It had a lower overall military strength than present. It's technology was barely any better than present.

The high point of human civilization was during the Dark Age of Technology, and even then it peaked with the creation of the STC systems. In fact, the STC's are one of the main reasons why human technical development went into decline, having machines make everything for you leaves little need to knowing how to do anything yourself. 

There are several possible scenario's in which things can get better for the Imperium of Man, but almost all of them require a major social and ideological shift, not only in Humanity, but in the Eldar, Tau and many minor species. 

Ironically, the Emperor isn't the answer here, since his rebirth would likely destabilise the Imperium, something extremely dangerous given the current delicate balance of power in the galaxy. Really, the Emperor's time is done, and the only way to actually save humanity is for it to stop following him.