how different would the 41st millennium look if:
well, if in those days before the emperor and the Imperium, when man first began exploring the stars and colonizing worlds. if their first contact with the sentient beings of the warp actually went rather well...
I mean look at it like this: humans, (prior to the imperium) have an innate sense of curiosity. So long as the denizens did not show immediate hostility the humans would probably want to ask questions.
As for the denizens of the warp , well why should they show hostility? for them they have been nurished by and captivated by the countless flickering lights of the warp, which these new creatures, these "humans" seem to be connected to.
not to mention they have a common hatred of the Eldar who have shown countless times their hatred of both species.
Now obviously First contact went horribly wrong considering that humans and the denizens of the warp want to destroy each other.
But honestly think about it... what if things went differently and the creatures of the warp ended up humanity's one and only true 'friend' among the stars.
and please suspend your inherent need for grimderp when contemplating this.