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.
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.
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 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.
In 'deku meets future deku fanfiction', perhaps the future Deku warns the present one about upcoming threats. He could tell him about villains he'll face in the future and how to defeat them. This would give the present Deku an edge in his battles and also in his growth as a hero.
One possible plot could be that Villain Deku is on a mission to undermine the hero society and crosses paths with Hero Deku. They engage in a fierce battle where Villain Deku taunts Hero Deku about his ideals. In the end, Hero Deku manages to get through to a small part of Villain Deku's conscience, leaving him with some doubts about his villainous path.
Another plot might involve the older Deku being from an alternate timeline. He shows the younger Deku different ways he could have developed his skills and relationships, which makes the younger Deku reevaluate his own path as a hero.
In the manga, Deku experiences various ups and downs. He not only becomes stronger physically but also matures emotionally as he takes on greater responsibilities. There are also many unexpected plot twists that shape his journey.
It could lead to a completely different character dynamic and story arc. Maybe she'd face different challenges and have a unique perspective on being a hero.