Instead of being deemed All Might's successor, All Might criticizes Izuku's actions. All for One, monitoring All Might, sees this and decides Izuku could be a viable "Plan B" if Tomura fails. Izuku is then taken as All for One's second apprentice.
At the end of the Hunt, the Hunter has submitted to Gherman's mercy. He awakes on March 26th 1994 in New York whereupon Behemoth attacks. During the attack he fights Behemoth and is discovered by the Triumvirate.
During his battle with All Might, All for One sends Izuku, All Might, and several other heroes to Brockton Bay with a new quirk he recently acquired.
What if the Doom Slayer was one of the Warp Gods the Emperor chose while creating the Primarchs and if Argent D'Nur was a lost human colony half way inside the Eye of Terror? What would change? How different would the Imperium end up by the fortieth millennium?