Well, it depends on different fanfictions. Some might have Madara being reborn with all his former power and knowledge, leading to epic battles against new foes or old rivals reimagined in new scenarios.
There can be really engaging plots. In some 'naruto madara reborn fanfiction', Madara is reborn with a different mindset. He might start to question his past actions and try to make amends in his new life, which leads to a completely different path for him and the Naruto world around him. This can involve him forming new alliances, teaching the new generation of ninjas, and changing the course of the ninja world's future.
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 Madara fakes his death in the fanfiction after meeting Naruto. Naruto then has to deal with the chaos that follows as Madara secretly manipulates events from the shadows. Or, it could be that Naruto's presence causes Madara to have a change of heart, but then the ninja world doesn't believe that Madara has truly changed and they both have to fight against the world's distrust. There are so many ways the plot can take an unexpected turn in this kind of fanfiction.
One idea could be a story where Madara and Hashirama are reborn in the modern ninja world and have to navigate the new technologies while still having their old grudges and loyalties. They might initially be confused but then slowly adapt and perhaps even find a way to resolve their age - old differences.
One idea could be that Madara takes Naruto under his wing to train him in a different style of combat, leading to Naruto having a unique set of skills. This would change Naruto's relationships with his friends as they might be initially skeptical.
Well, a cool plot twist might be that Fem Naruto has a secret lineage related to Madara's own clan. Maybe she discovers this and it makes Madara question his long - held beliefs and grudges. Or perhaps in the fanfiction, Fem Naruto is able to use a new form of ninjutsu that is a combination of Uzumaki and Uchiha techniques, which she uses to outwit Madara in a crucial battle. It could also be that Madara's real goal was to actually find a successor like Fem Naruto all along and his actions were just a test to see if she was worthy.
In some fanfictions, there could be a plot twist where Madara and Naruto end up switching sides for a brief moment. Maybe Madara sees the error of his ways due to Naruto's unwavering belief in friendship and peace, and he helps Naruto defeat an oncoming threat, only to then go back to being the antagonist. Or perhaps Naruto discovers that he has a hidden connection to Madara's past, something that changes the whole dynamic of their battle and leads to Madara's defeat in a very unexpected way.
One interesting plot idea could be that the reincarnated Madara has to deal with modern-day ninja politics. Since he was from a different era, his views on how a ninja village should be run might clash with the current system, leading to all sorts of power struggles and attempts to reform the village.
One idea could be Naruto growing up with the Uchiha heritage and having to deal with the expectations and prejudices that come with it. Another could be his relationship with Sasuke being even more complex as they are now related in this fanfiction universe. Maybe Naruto discovers some long - hidden secrets about the Uchiha clan from Madara's past that could change the course of the ninja world.
One possible interesting plot could be Madara, in his Edo Tensei form, somehow regaining his full power and challenging Naruto not in a fight to the death but in a battle of wits. Naruto, on the other hand, tries to find a way to either seal Madara permanently or make him see the error of his ways. For example, Naruto could use his talk - no - jutsu to try and convince Madara that his quest for power has only led to destruction.
Well, a major theme is the exploration of the past and present. Since Naruto is reborn as Madara, he has access to Madara's memories, so there's a lot of looking back at the history of the ninja world. Also, the theme of friendship is present but in a new light. His old friends now see him as Madara, and he has to figure out how to maintain those relationships. And power dynamics are a theme too, as his new power level changes how others interact with him.