There are indeed. Some fanfic writers find the idea of All Might being Deku's dad an interesting twist. It can lead to some really heartwarming and intense storylines. For example, in these fanfics, All Might might have a more personal reason for choosing Deku as his successor, and Deku could inherit not only One For All but also some other traits or legacies from All Might in a more family - related way.
He might cry a lot. Deku is very emotional, and being cheated on would be a very painful experience for him. He could also get angry, which is rare for him, but this situation would be so unjust that he might not be able to hold it in. He'd probably start to question his self - worth too, thinking that there was something wrong with him that made his partner cheat.
In most of such fanfictions, they might create evidence like hidden past connections. For example, AFO could be secretly involved in Izuku Midoriya's (Deku) family history. Maybe he had a relationship with Deku's mother long ago, which led to Deku's birth. But it's all fictional speculation within the fanfiction world.
One common plot twist is that Deku discovers the truth about his father in a moment of great crisis. For example, when he's facing off against AFO in a big battle, AFO reveals the truth to him, which completely shakes Deku's world view.
Deku might be initially shocked. He's used to Bakugou's aggression, but an actual injury is different. He could be in pain and disbelief at first. Then, he might try to downplay it, especially if he doesn't want to cause trouble for Bakugou, because Deku is generally a very forgiving character.
She might be devastated. Bella could go into a state of shock initially. Then, she might feel a deep sense of loss and loneliness. She could also be filled with anger towards the cause of her father's death.
Well, some might be really amazed. They could see a new side of Deku that they never knew before. His singing might show a different level of confidence or a hidden talent, so they would probably cheer him on.
One interesting plot point could be All Might revealing the truth to Deku later in the story. It would be a very emotional moment, with Deku having to reevaluate his entire relationship with All Might. Another could be Deku inheriting not just One For All but also some family heirlooms or special abilities from All Might's side of the family.
Bakugo would likely be furious. He'd feel betrayed and would probably be the first to attack Deku, yelling at him about how he's thrown away everything they've worked for. Uraraka might be more empathetic. She'd try to understand what drove Deku to become a villain and hope there's a way to bring him back to the side of good.
His dad might be initially shocked but quickly become accepting. After all, he is a god and has seen many strange things. He would probably use his power to make Percy's life easier during the transition, like providing resources or protection.