One idea could be exploring a character's backstory who was wrongly imprisoned in the Blood Prison. For example, a shinobi who was framed by a powerful enemy and has to prove their innocence while dealing with the harsh conditions and dangerous inmates in the prison.
Well, a plot idea could involve Haren being framed and sent to the Blood Prison. Once there, he uncovers a conspiracy that reaches outside the prison walls. He then has to find a way to expose it while dealing with the prison's internal power struggles and dangerous inmates. This could involve using his wits and any latent ninja skills he might have.
One idea could be the main character having to protect the harem members within the dangerous Blood Prison setting. Maybe there are secrets about the prison that only the harem members know, and they rely on the main character to uncover them while fending off other threats.
Another plot could involve Naruto having to deal with a hidden power within the Blood Prison that is starting to corrupt the inmates and guards alike. He has to find the source and stop it. For example, there could be an ancient curse that was placed on the prison long ago and is now awakening. Naruto, with his strong will, tries to resist the curse's influence while also finding a way to break it for everyone else.
In some fanfictions, Naruto might gain an unexpected power - up while in the Blood Prison. This new power allows him to not only escape but also take on the forces that put him there in the first place. It could be related to his Uzumaki heritage or a new form of sage mode.
A really interesting plot could be a time - travel fanfiction. Naruto and Temari are sent back in time to a crucial moment in the history of their villages. They have to work together to make sure the events unfold in a way that doesn't change the future too drastically, but also helps to prevent some of the darker events that occurred in their past.
One interesting plot idea could be exploring different character backstories more in - depth. For example, delving deeper into the history of side characters like Kiba and his family. Another could be a rewrite of major battles with different outcomes, like if Sasuke had lost his fight with Itachi.
In this type of fanfiction, there could be a plot where a strange disease starts affecting ninjas' chakra systems. Naruto has to lead the search for a cure, which takes him on a journey across the ninja world. Along the way, he discovers secrets about the origin of chakra that were long forgotten. This could also lead to some interesting character development for other supporting characters as they face this new crisis together.
One interesting plot idea could be Naruto's son, Boruto, facing a new and powerful enemy that has ties to an ancient evil from the past that was thought to be sealed away long ago. This enemy could be manipulating the new generation of shinobi in the Hidden Leaf Village, causing chaos and mistrust among them.
One interesting plot twist could be that Naruto discovers a hidden power within the Blood Prison that is related to an ancient clan. This power gives him an edge in battles and also helps him to uncover secrets about the prison's origin.
Maybe a fanfic where Naruto himself is imprisoned by an enemy village that has a false accusation against him. He then tries to break out not just for his own freedom but also to prevent a war that the false imprisonment might trigger. He could inspire other prisoners to join his cause through his unwavering spirit.