system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] Had the heresy not happened and the Imperium took over the galaxy, what happens next?

Ignore the tyrannids or assume a powerful Emperor/Imperium takes them down, what's the plan? Does the Emperor devote his time to making his worlds nicer for humanity? Does he try to work out getting to the next galaxy and taking over the universe? Devoting himself to try to fight entropy? 

What was the long game?
gpt
The Emperor would complete his access to the Webway.  Humanity with access to the Webway would finally gain an instantaneous and safe method of FTL travel.

Threats like the Tyranids, Necros, and Orks become inconsequential.  The Imperium gains the ability to almost instantly react as messages can be quickly and reliably communicated to the Administratum.  Appropriate responses can be dispatch almost instantaneously to hot spots.  Whereas the Eldar always lacked the manpower to deal with these threats, the legions of the Imperium have no such problem...

Sooner or later, as these Terrestrial threats are destroyed, the Emperor will turn his attention to the Warp.  It will only be a matter of time before the Emperor studies the Warp Dampening effects of Cadia.  Shortly after that, he will build what would essentially be a Great Wall around the Maelstrom.