One common theme is the exploration of a new world. The underworld in Naruto fanfiction is often a mysterious and dangerous place. So, Naruto has to explore it, learn its secrets, and find his place. Redemption can also be a theme. Sometimes Naruto tries to redeem characters from the underworld or even the underworld as a whole. And of course, friendship. He makes new friends in the underworld, which is different from his Konoha friends, and these relationships help him in his adventures.
In some fanfictions, there might be Naruto - related shops near areas that could be considered 'underworld - like'. However, it really depends on the creativity of the fanfiction author. Some might create such unique settings for the sake of plot twists or to introduce interesting new elements into the Naruto universe within the fan - made story.
One idea could be that Amelia is a new arrival in the Naruto universe's underworld. Maybe she has some unique powers that draw the attention of the various criminal elements, including some of Naruto's foes. Naruto, being the hero, gets involved to protect her and discovers there's more to her than meets the eye.
It's possible that some new characters are introduced in this fanfiction. They could be powerful underworld figures that Naruto has to deal with. These characters could have their own motives and backstories, and they would interact with Naruto in various ways, either trying to ally with him or overthrow him as the king of the underworld.
In this fanfiction, Naruto might leave the underworld because he has a mission to fulfill in the real world. Maybe he has to stop a new threat that endangers both the living and the underworld.
There could also be original characters created specifically for the fanfiction. These characters could be hybrids of the Naruto and Selene universes, perhaps having a mix of ninja abilities and Underworld powers. They could add new dimensions to the story and help drive the plot forward, whether as friends or foes to Naruto and Selene.
Sasuke could also be a main character in this fanfiction. He might be interested in the power of the lycans, seeing it as a way to gain more strength for his own goals. His brooding nature and his search for power could lead him into the lycan underworld, where he has to make choices about whether to ally with the lycans or use them for his own purposes.
Naruto's powers would likely be seen as very strange and powerful in the underworld. His ability to create shadow clones might be mistaken for some sort of dark magic at first. His rasengan, a powerful and concentrated ball of chakra, would be seen as a formidable weapon. The underworld beings, who may be used to different forms of magic and power, would either be in awe or fear of Naruto's abilities.
Shikamaru could also be an ally. His intelligence would be invaluable in planning strategies and outwitting the more cunning criminals. He could help Naruto figure out the complex power structures within the underworld and come up with plans to either take them down or use them to Naruto's advantage.
Naruto himself is often very popular. He brings his positive energy and never - give - up attitude to the underworld setting, which can create a lot of interesting storylines. His ability to make friends and his powerful jutsu make him a central figure.