system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] Say Horus is never injured, the Heresy never happens, and Horus decides to integrate willing xenos species into the Imperium after contact with the Interex. What does the galaxy look like now?
gpt
You're essentially asking a Best-possible scenario... But I'll bite.

Without the Horus Heresy, the Imperium continues to spread, solidifying the Emperors control over the Galaxy. Invasion of the Webway increases Human FTL and trade, but bring the Imperium more in conflict with the Dark City. 

Realising they have no choice, many Eldar join the now more tollerant Imerium, their technology and that of other Xenos causing tension with the Adeptus Mechanicus. The Emperor is forced to enact his final solution and anihilates the Machine Cult. This heavily damages Imperial production, but Eldar and Interex technology quickly closes the gap.

The use of the Webway, and the psychic knowledge of the Eldar rapidly accelerates humanities Psyker manifestation, making Psykers far more common and safe.

The increased stability and peace weakens the Chaos Gods. The stability of the Eldar may even cause their gods to re-exert themselves, tearing Slaanesh appart from within.

The Eldar warn/manipulate the Emperor into at striking against the sleeping Necrons. The Rogue C'tan are hunted and destroyed. The Necron Dynasties are shattered before they can awaken.

The Tau may not ever come to exist. If the Eldar and Interex can convinced the Imperium to spare them, they may develop and one day join the 'Galactic Imperium'.

The Orks cannot be erradicated, but they are once again contained.

When the Tyranids arrive, they find a far stronger, though also far more populated galaxy. They arrive in greater numbers than before. The enevitable outcome is still uncertain.