Well, a plot could involve the Akatsuki somehow making a deal with an underworld lord. Naruto discovers this and tries to stop it. He teams up with some unexpected allies from the underworld who also oppose the Akatsuki's actions. Along the way, Naruto has to learn about the unique powers and rules of the underworld to be successful in his mission.
One possible plot could be that Naruto is somehow transported to the underworld. There, he meets a powerful underworld character who has a grudge against Konoha. But Naruto, with his determination, tries to make peace. Along the way, he discovers a new form of chakra related to the underworld's energy. And as for the 'lemon' aspect, perhaps there could be a budding romance between Naruto and a female underworld dweller where they share some tender, romantic moments.
One idea could be that the lycans and vampires in the Underworld are being haunted by some strange demonic presence. The Ghost Rider, sensing this unholy chaos, rides into their world. He could initially be seen as a threat by both sides but then they realize he's there to fight a common enemy, like a powerful demon that wants to use the eternal war between the lycans and vampires to its advantage. The Ghost Rider's hellfire powers could be a new and terrifying element in the Underworld's dark and violent setting.
A plot might involve Sailor Moon herself getting lost in the Underworld. As she tries to find her way back, she discovers that there are some similarities between the magic in the Underworld and her own powers. She could meet a character in the Underworld who has a connection to her past or her future. Together, they start to uncover a mystery that links the two worlds and has the potential to either save or destroy both.
One interesting plot could be that Naruto is somehow transported to the underworld. He meets various underworld characters and tries to use his ninja skills to survive. Maybe he has to fight against underworld demons using his shadow clones and rasengan. He could also try to find a way back to his own world while learning about the power dynamics in the underworld.
The plot could involve Hermione being sent to the Underworld as part of a punishment or a test. Once there, she discovers that there are secrets in the Underworld that could have a great impact on the wizarding world. She teams up with some of the more benevolent underworld beings, like Charon the ferryman, who helps her navigate the different regions in exchange for her knowledge of magic. She then has to face off against the more malevolent forces in the Underworld, like Hades himself, to prevent him from using a powerful spell she brought with her accidentally to take over the wizarding world.
Well, perhaps in this fanfiction, the SAO system is corrupted by an Orochimaru - like figure. He wants to use the virtual world to gain power and immortality. Naruto and his friends, along with the SAO players, have to team up. Naruto's positive attitude and never - give - up spirit inspire the SAO players. They all work together to defeat this new threat, with Naruto using his Rasengan in new and creative ways within the SAO world.
A plot could involve the Killzone characters getting access to chakra. Some of the Killzone soldiers might start to learn ninjutsu and this causes chaos in their own military structure. Meanwhile, Naruto has to figure out how to get back to his own world while also dealing with the new threats that this crossover has created. Maybe he teams up with some of the more honorable Killzone soldiers to find a solution.
A plot might involve Naruto being sent to Bayonetta's world. He'd be out of his element, but his determination would shine through. Bayonetta could mistake him for a strange demon at first. However, they soon realize they can help each other. Naruto's chakra could interact in unexpected ways with Bayonetta's magic. Together, they could face off against Bayonetta's foes, like the angels, with Naruto's Rasengan combined with Bayonetta's Witch Time abilities making for some epic battles.
In a Naruto and Spiderman crossover, it could start with Spiderman noticing some strange energy signatures in his city. These turn out to be Naruto's chakra flares. Naruto is there looking for a way back to his own world after being accidentally sent to Earth - 616 (the Marvel Universe). They meet and at first don't understand each other. However, when they see a common enemy, a group of ninjas - like mutants who are causing chaos, they decide to work together. Spiderman uses his agility and web - shooting while Naruto uses his jutsu to defeat the bad guys.
One possible plot could be that Naruto and his team are on a mission in a strange new land that turns out to be the world of Claymore. They encounter Claymores, the half - human, half - yoma warriors. Naruto, with his never - give - up attitude, tries to understand the complex relationship between humans and yoma in this world. He could end up befriending some of the Claymores and together they fight against a powerful common enemy that threatens both their worlds.