system
Given the following post, respond as if you're commenting on Reddit:
human
[40k/Trek] What would it take to transform the Federation into the Imperium? Politics-wise.
gpt
The answer to this question comes from 40K itself, since humanity's empire during the Dark Age of Technology was almost identical to the Federation in Star Trek. 

What would need to happen first is some cataclysmic conflict erupting across the entire Federation, severely damaging its infrastructure and its economic and social systems. Perhaps a "hologram rebellion" started by an Emergency Command Hologram that *Voyager's* Doctor helped to create after Voyager returned to the Alpha Quadrant. 

Next, humanity's ability to quickly and safely travel between planets would need to disappear. Perhaps it would be discovered that *all* travel at Warp speed destroys the fabric of subspace, and thus can't be used except in extreme emergencies, while at the same time quantum slipstream like the Borg use is still so complex that Federation scientists don't even hope to do more than scratch the surface of its potential for a full millennia. 

Following that, communications between planets would have to disappear. Perhaps subspace is so badly damaged by Warp travel that subspace transmitters can no longer reliably get a message from Point A to Point B. 

After that, some alien races would need to invade Earth a few times and go on a few looting, pillaging and burning sprees. The Klingons would be a likely candidate, as would the Orions and the Gorn. 

And after all that, there would need to be some sort of genetically-engineered superhuman who had lived in the shadows for untold ages during the heyday of the Federation. This being would finally reveal himself to Earth and slowly build up an army that would unite it under his rule, with the eventual goal of reuniting all of the old Federation territories. An Augment like Khan is probably the best bet here, a survivor of the Botany Bay who didn't die with him chasing after Kirk.

Then add 10,000 years of hate, ignorance, superstition and blind faith in humanity's manifest destiny to conquer the stars, and you'll get something that isn't *quite* like the grim darkness of the 41st Millennium, but is pretty darn close. 

Somewhere in a beautiful and cheerful corner of Heaven, Gene Roddenberry is weeping tears of horrified despair at this perversion of his beloved universe and its noblebright ideals...