MHA x Ben 10 MR/OC

In this book you the reader is the son of David Sheild, and brother to Melissa Sheild.
As you know All Might and David are best bros, which means All Might is essentially your uncle.

You're born with multiple Quirks, but having so many in one body would be a catastrophe. The user's body could become unstable. So as a safety precaution David designed a device capable of organizing, and utilizing each Quirk to their fullest potential. The device also grants them suitable physical forms.

In this story David knows about OFA.
He and All Might asked Melissa if she wanted to be his successor, but she declined, wanting to continue learning under David.

So you were the next best option. They were against it at first, but you managed to convince them. After having reluctantly agreeing, they made you promise that you'd train with the device, your Quirks, and OFA.