What if...
The Primarchs were found in a different order?
The one that I fnd most interesting to consider is the idea of Magnus the Red being found first and Motarion being found last. I would imagine that this would lead to the Librarium's being well established by the time Mortaion is found and the Council of Nikea probably not being a thing.
However I can't see Mortarion being a fan of psykers after being subjected to Barbarus for even longer. I would think that he would likely try to separate his legion (Death Guard) from the Imperium and would likely become a rallying point for any with a distrust f the psyker.
Concurrently, if the psychic powers of the Imperium were guided by Magus from their inception, it Imperiums psychic powers would probably be alot more like sorcery than the more constrained powers the Imperiums currently exhibits.
*Bonus - Angron doesn't get found until after his dies with his gladiator brothers on Nuceria. Afte finding out about this the War Hounds (World Eathers in main 40K) Go on a ramgape and murder the world in their grief. But would they then stay as raging barbarians? Or would this drive them to emulate the 'champion of the enslaved' arcchtype that Angron was?
What 'what if' do you like to think about?