We're going to just be calling this the Lost Primarchs AU for the time being, because we don't have a better one yet, but here we are!
Primarch Ji-Yeong Stojanovska, leader and eldest "sibling" of the Order of the Solar Heralds. The Solar Heralds are a genuine monastic order, focused on a specific subset of their greater cultural religion - one that focuses on those who intentionally take upon struggle for those who cannot fight for themselves, one that offers community to even the most isolated. The order builds well educated, creative individuals, who are masters in at least two fields of combat alongside their non-combat fields of exploration. They represent a warm face and a helping hand in the worst of situations, whether that be the ravages of war or of natural disaster.
Ji-Yeong Stojanovska, having been scattered to the winds alongside their siblings, lived upon the fringest elements of the milky way galaxy. As a result, they were found by the Emperor very late. Late enough, in fact, that Ji-Yeong had worked with their fairly advanced society to carve out a small confederation of planets, alongside another primarch, and his sibling, Nemo. A confederation that was threatened by the encroachment of the Imperium.
When The Emperor showed up at "Philophrosyne," as the Imperium had marked it upon their astral charts, Ji-Yeong stood at the host of a collection of warriors. He looked upon the Emperor's host, the collection of Terran marines, and he soured. He had scene the Emperor's type before, and no predestination would shake him of this belief: The Emperor was a fool. And the Emperor could leave.
While the Emperor could have easily wiped out his wayward primarchs, this far out from the Imperium, the aberrant pair of mutant primarchs were easily banished. The legions that the Emperor had raised for Ji-Yeong and Nemo, intended for use in his grand crusade, were abandoned. Left behind. Without their Primarchs, they were simply unneeded baggage, and the Emperor left, back to his grand project, that would fail, for the simple fact that he refused to treat people as equals, as things deserving of respect and care and a space at the table.
There's more story to come, but this is where we're gonna stop for this post, because we don't wanna bog it down too much! But fundamentally, the idea was to explore a setting where the galaxy was not as bad as the imperium makes it out to be, and instead all of the "canon lore" is just propaganda by a decaying fascist empire to try and terrify people into orthodoxy.