Well, Naruto is the central figure when it comes to the training aspect. He has his ninja skills, his strong will, and his sense of justice. Fem Vali Lucifer, on the other hand, is an important character as she is the recipient of the training. There could also be other minor characters from either the Naruto world or the world that Fem Vali Lucifer comes from that might play a role in the story, for example, Naruto's friends who could offer some advice or Fem Vali Lucifer's companions who might have an opinion on her training with Naruto.
Well, it could be that Naruto, with his ninja skills and determination, helps Fem Vali Lucifer to master some unique powers. Maybe Naruto teaches her stealth techniques or how to better control her energy. He could use his experience from training in Konoha to design a special training regime for her.
One possible scenario in this fanfiction could be that Naruto uses his ninja training techniques to help Fem Lucifer develop new skills. Maybe he teaches her stealth, which she can then use in her own realm to gather information more effectively.
Perhaps the plot involves Naruto being summoned to Fem Lucifer's world. He then realizes that she needs training to deal with a new threat in her realm. So, he starts training her in various ninja arts.
Another theme is adventure. They might embark on quests together, like exploring ancient ruins or fighting powerful enemies. For example, in some fanfictions, they go on a journey to find a lost ninja art, which involves a lot of action and discovery.
One popular type of fanfiction about Vali Lucifer in Highschool DxD could be his power development. Writers might explore how he further hones his abilities, perhaps by training in new, secret locations. This could involve him facing off against powerful, previously unknown foes that push his limits and force him to unlock new levels of his power.
There's also a fanfiction where Fem Vali becomes Naruto's ally in a mission to save the Hidden Leaf. In this story, their different skills and personalities complement each other, creating a unique bond. It delves deep into how their relationship develops over time, with some great action scenes and emotional moments.
Character development is key. You want to see how the two very different characters of Vali and Lucifer are portrayed and how they change over the course of the story. For example, if Vali is initially distrustful of Lucifer but then comes to respect him.
One possible theme is the clash of different worlds. Naruto comes from a world of ninjas and chakra, while fem Lucifer is from a more otherworldly, perhaps demonic realm. Their relationship can explore how two very different beings interact and find common ground.
Another great piece is a more comical take on the crossover. It shows Vali and Lucifer getting into all sorts of hilarious misadventures in a modern - day city. They have to deal with things like traffic, noisy neighbors, and trying to hide their true identities while also trying to figure out why they've been brought together in this strange new world.