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 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.
One common theme is power. Since Vali is a powerful character, many fanfics explore how she uses or gains more power. Another theme could be identity, as she might struggle with who she is in different situations in the fan - created stories.
Well, it depends on what you consider 'good'. If you like action - packed stories, you might find some where female Vali Lucifer is in the middle of some epic battles. There are also ones that explore her character development, showing how she grows and changes over time. To find them, you can try using specific tags in addition to the character name when searching on fanfic platforms, such as 'adventure' or 'character growth' along with 'female Vali Lucifer'.
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.
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.
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.
A plot idea could be Vali being sent on a secret mission by the Devils. He has to infiltrate an enemy stronghold that is rumored to be developing a weapon that could destroy the balance between the three factions. Along the way, he teams up with some unexpected allies, including some from the Angel faction who also don't want the balance to be disrupted.
There's a fanfic titled 'Unlikely Alliances' which features an OP OC and FEM VALI. This fanfic is notable for its detailed world - building. The author creates a complex universe where the OP OC, with their extraordinary abilities, must team up with FEM VALI to face a common threat. The story delves deep into their individual backstories and how they grow as characters throughout their journey together, which makes it a really engaging read.