@evilminji recently made a Post about DP x MHA and I'm in love with the idea of those 2 fandoms together. (I'm gonna tag this dpxdc just to spread it around but there won't be much dc in it. Maybe a few crumbs here and there)
So! Quirks came into existence the same day Danny became a Halfa. He wasn't directly responsible for it, but when the Portal opened it released a truly Massive amount of Ectoplasm into the world at large. The Dr's Fenton didn't notice it at first, because Amity Park had always had that level of Ectoplasm and all of their Sensors were within the City Limits.
The Ectoplasm released onto the world quickly and integrated with multiple Humans across the Planet, and eventually enough Ectoplasm gathered in a single Human to form the first Proto-Core, aka Quirk. That human was the Glowing Baby.
This phenomenon spread across the world as the situation in Amity Park devolved. The GIW knew for certain that these "Meta Abilities" were the result of Ectoplasm Contamination, and doubled down on Amity Park to try and fruitless contain the spread. Not that it would have worked, the Portal opening had sent a Shockwave across the fabric of Reality, creating mini-portals that continued to feed Ectoplasm to the Earth.
Eventually the day came when Amtiy Park was Evacuated from the Mortal Realm by Clockwork for it's own Protection. It's disappearance was blamed on a newly awakened Quirk destroying the entire Town, which was used to feed the flames of hatred in the Early Dawn Era.
Then one day, Amity Park returns. In Japan of all places.
Clockwork had dragged the entire Town through Time, to an Era that would accept their abilities. It was placed in the countryside of Japan. Clockwork had considered putting them in a dimension of Heroes, bur decided that would like to stay in their home dimension for now.
Thats the basic world building, but there are other parts we can expand upon in this AU
All for One was the first True Core in the 1st Gen of Quirks, but he was born with a Defect that made him unable to generate his own Ectoplasm. He felt a Hunger that could never be satiated, and eventually found a way to feed that hunger by absorbing the Proto-Cores of other people into his own True Core, stealing their Quirks.
One For All was the Opposite, born as an empty True Core but without the insatiable Hunger that his brother had. Then AFO force fed him a Stockpile Core and it fused with his own, giving him the power to Generate and Stockpile his own Ectoplasm.
He also had the power to Pass On his Core to another Body, basically just passing on his power. Then his Successor did the same, and their successor, and their Successor. Eventually we reach the 9th Holder of OFA, and the Singularity.
In this world, the Quirk Singularity isn't just an event where Quirks grow too powerful for their Bodies to handle, it is an event where the Proto-Cores of Quirk Users experience an Apotheosis, and change from Proto to True Cores. True Cores which the Human Body can't handle having.
But it's different for OFA. It was a True Core from the beginning, born within a Human Body, and it has the power of 6 other Proto-Cores within itself all ready to Apotheosis into True Cores themselves. When OFA is passed onto zuku Midoriya, it floods him with 7 Cores worth of Ectoplasm, and turns him into a Halfa.
When Danny and the rest if Amity Park reappear in Japan, they find not only a world populated by mostly Liminal Humans, but also a Powerful Halfa who doesn't even know he is a Halfa. Maybe even 2 if All Might also experienced the same Apotheosis?
