I don't know if I'm just being cynical but I find it hard to believe the Emperor could not see the Heresy coming.
I can't help but think that the Emperor orchestrated the events that lead to the Heresy, to bring about his ascension to godhood, without proclaiming himself a god. If he had just told the people he had found/conquered to worship him or else they would have turned on him. He needed humanity to turn to him in their darkest hour.
Here is what I think happened.
The Emperor unites the scattered enclaves of humanity under his rule.
All these groups of humans have different beliefs so he "encourages" them to become atheist.
Then he disappears.
Then the Heresy starts creating a horrifying new threat for humanity.
People start calling out to the benevolent but absent father figure that is the Emperor.
With his final "sacrifice" and the psychic energy of so much of humanity focused on him his ascension is complete.
Now I admit I only read the first 3 Horus Heresy novels and I am no expert but is there any evidence that the Emperor set up the dominoes that lead to the Heresy or was he, the oldest and wisest "human being", just blind to the shortcomings of his sons?