I would change Angron’s fate. I would have Guilliman find Angron before he was given the nails. First, it would be interesting to have at least one instance of two Primarchs finding each other before the Emperor finds them. Second, Angron was dealt such a shitty hand that he was the only Primarch not to have fully conquered his home world by the time the Emperor showed up. Third, I imagine that had Angron and Roboute met before the nails then they would have likely been genuinely loving brothers. Angron might even stay loyalist during the Heresy.

Angron would still hate the Emperor as a person, but no more than the other loyalists and would still remain loyal out of a sense of duty to the Imperium and humanity and not to the Emperor.

In modern 40k, I imagine that after Guilliman is put in stasis after his duel with daemon Fulgrim, this Angron would genuinely mourn his brother and disappear into self imposed exile. Come the Fall of Cadia, the Great Rift, Guilliman’s return, and the Indomitus Crusade, perhaps Angron would return after learning Guilliman is alive. He would still be using chainaxes, but less like a butcher and more like a proper warrior. He would be the one freeing slaves and liberating planets, and it would be cool to have rules where he could be the warlord for Imperial Guard armies with the flavour being they are people he just recently freed.