I think young Sasuke would have a very intense reaction. He was so consumed by his hatred for Itachi at that young age. A Sasuke vs Itachi fanfiction would be like a trigger for him. He would be eager to see how the fictional fight is portrayed. If Sasuke in the fanfiction is shown as strong, it might fuel his own ego and make him strive even harder to become powerful. However, if the fanfiction shows Itachi as superior, it could enrage him. He might feel that it undermines his own goal of defeating Itachi and he might even want to find the writer to correct their 'wrong' view.
Well, one possible outcome could be that Naruto and Sasuke manage to defeat Madara through their combined strength and newfound powers. They might use their signature jutsus in a coordinated way that Madara can't counter.
In such a fanfiction, Naruto and Sasuke might win. They are both extremely powerful and their combined strength, like Naruto's Rasengan and Sasuke's Chidori, could overpower Madara.
He might feel anger. Since he was so set on revenge against Itachi, any portrayal that didn't match his view of himself as the victor could make him angry.
Sasuke could also be the one. With his Sharingan and Rinnegan, he has unique abilities. He might use his Amaterasu in a very unexpected way, like creating a large - scale trap for Madara using the black flames. His intelligence and cold - blooded nature in battle make him capable of making a decisive move.
There's also 'Itachi and Madara: The Unseen Alliance'. This fanfiction imagines a situation where Itachi and Madara form an alliance for a greater good, against some common enemies that the original Naruto series didn't explore. It's full of twists and turns and the character interactions are really engaging.
I really like 'Eternal Connection'. In this fanfic, Madara and Itachi are brought together in a time - travel scenario. It's a really interesting concept as they both have to deal with the knowledge of the future and past while also developing their relationship. It's full of action and some really touching moments.
In this kind of fanfiction, Itachi might use his powerful Sharingan abilities. He could perhaps create illusions to mislead Orochimaru and his minions, then quickly snatch Sasuke away. For example, he could make Orochimaru think that Sasuke is in a completely different location while he secretly approaches and rescues him.