In some interpretations, it could be another powerful ninja who has a secret relationship with Madara. Maybe it's someone from a rival clan, which would add a lot of drama and conflict. For example, if it were a member of the Senju clan, considering the long - standing feud between the Uchiha and Senju.
In most fanfictions, evidence is often created within the fictional narrative of the story. It could be things like hidden backstories, special powers passed down that suggest a connection, or certain encounters between Madara and Sakura's mother. However, in the original Naruto series, there is no such relation.
One common plot twist could be that Madara had his memories wiped and didn't know about Naruto being his son until some fateful encounter. Another might be that Naruto's mother was part of a secret mission involving Madara.
Fanfictions often take creative liberties. In the case of 'madara is naruto's father fanfiction', the evidence might be as flimsy as a shared look or a similar power that the author forces to be a connection. But remember, this is all in the realm of fan - created stories and has no basis in the real Naruto lore.
A common plot device is the misinformation or false memories within the story. For instance, Sakura might have been raised believing one thing about her father, but then through a series of events in the fanfiction, she discovers that Madara is her real father. Another device could be the use of a special bloodline or power that only makes sense if Madara is her father. This power then becomes a central part of the plot as Sakura tries to master it and understand her true identity. Also, the idea of a fated encounter between Madara and Sakura's mother, which was kept secret for some reason, is often used to drive the story forward.
In such fanfiction, Sakura might be depicted as more alluring and perhaps more bold in her interactions with Madara. Madara could be shown as having a certain charm that draws Sakura to him.
Well, it could be through his powerful ninja techniques. Maybe he uses his Sharingan to paralyze Sakura, or perhaps he physically restrains her with his great strength. In fanfictions, there are many creative ways for such a scenario to play out.
Madara could also save Naruto by using his knowledge of the ninja world. Maybe he has some inside information about a plot against Naruto and he intervenes in time to stop it. He might have discovered a secret plan by an enemy group and takes action to keep Naruto out of harm's way. This could involve manipulating events behind the scenes or directly confronting the threat.
Naruto is often paired with Madara in some fanfictions. Their interactions can be really intense, as Naruto tries to either convince Madara to change his ways or fight against his reborn form.
Perhaps he sees her as a threat. Sakura is a skilled kunoichi, and Madara may want to prevent her from interfering in his plans. So he restrains her to keep her out of the way.