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.
In a hermione and the underworld crossover, Hermione might be transported to the Underworld due to a spell gone wrong. She realizes that the Underworld has a different kind of magic system. She starts to study it and finds that there are similarities between the magic there and in the wizarding world. She uses this knowledge to bargain with the gods of the Underworld. She offers to help them solve a problem in their realm in exchange for a way back home. Along the way, she also helps some lost souls find peace, which earns her the respect of some of the Underworld deities.
Hades would be a likely main antagonist. He rules the Underworld and might see Hermione as a threat or an opportunity. If he thinks she has something valuable, like her knowledge of magic, he could try to keep her trapped in the Underworld. Or he might oppose her if she's trying to change something in his domain that he doesn't want changed.
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.
A plot could involve Hermione being chased by some dark forces in the wizarding world and accidentally using a spell that transports her to Sam's world. Sam's pack senses her magical presence and is on guard. But Hermione quickly shows that she means no harm. They start to communicate, and Hermione discovers that there are similarities between the magic in her world and the spiritual connection that Sam's pack has with their land. Together, they work towards creating a magical barrier to protect both their worlds from any potential threats that might cross over from one to the other.
One possible plot could be that Hermione, as a Lycan, struggles to hide her identity within the Hogwarts community. Fleur, being perceptive, might notice something off about Hermione. Maybe Hermione is afraid of being discovered and ostracized, while Fleur, having her own experiences with being different (as a Veela), tries to help Hermione come to terms with her new lycanthropy. They could form a strong bond as they both deal with the challenges of being 'other' in the wizarding world.
Some possible plot ideas include Hermione being mistaken for a powerful Grounder because of her magic. She could then use this to her advantage to broker peace between the different factions. Or maybe she discovers a magical element in The 100's world that she didn't expect, like a magical artifact left over from a pre - apocalyptic civilization. This could lead to a race among the different groups to obtain it, with Hermione in the middle trying to ensure it's used for good.
Well, it could be about Hermione somehow getting involved with the Quileute culture. Maybe she travels to their territory and learns about their traditions, magic, and way of life. It could also involve her forming relationships with the Quileute characters, perhaps even falling in love with one of them.
One possible plot could be Hermione using her Goth style to rebel against the traditional Hogwarts norms. She might start a club for those interested in the Goth subculture within the school.