system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K]What would have happened to mankind if the age of strife never took place?

Feeling waves in the warp the emperor foresees the fall of the eldar and the resulting chaos. He takes control and unifies mankind under His rule.
Psykers are prosecuted wherever they are found and all focus is put on alternative FTL technologies independent of the situation in the warp. The intelligence of the men of iron is restricted so they could never develop the thought of rebellion. 
When the warpstorms are beginning to set in mankind is thoroughly prepared and can continue their conquest of the galaxy unhindered under the rule of its glorious emperor.
Timeskip to M42. How is the current situation of the empire?
gpt
Likely the Space Marines would not have been created, though demi-god's they were created for the "Only War" situation of the crusade, even in original it was likely Empy planned for them to all die during the crusade or sent on suicide missions. They had no place in his ideal imperium.

With alt warp travel id imagine the human empire being smaller and more concentrated, focusing on Terraforming everything they encounter. Also lots of extensive human modification, biological and mechanical. Quality of life for an avg human would be much much higher. Plasma tech is mastered everything is kept at or above archaeotech level. This all presumes his plan succeeds now though, more likely it spirals out in another way and we see a larger mechanicum cult emerging and committing their own Horus Heresy.

Chaos would look for an alternative vassal, maybe a young Tau race pre-civilization and ethereals. More likely another unknown lesser race that after 10k yrs Chaos manipulation would have some threat. During the Horus Heresy many aliens were encountered that worshiped Chaos, a united xeno-chaos empire would be likely without early intervention.

Tyranids since they are coming outside of galaxy will likely arrive exactly as before, human resistance would be strong but their smaller empire makes them vulnerable to a swarm. Probably a stalemate just like in original. Maybe Tyranids would arrive later, some say the Tyranids are attracted to the psychic imprint of mortals and its associated biomass.

Orks will zuggin and zoggin as usual but in a larger playing area. No real persecution to stomp out rampant growth but also no incentive for a WAAAAGGGHHHH!!!!!!