She could be a foreign ninja. For example, she comes from a far - off land where they have different fighting styles. Her skills might be based on flexibility and speed, inspired by the lemons from her homeland which are known for their agility - enhancing properties. She could join Naruto's team on a diplomatic mission to her homeland to form an alliance.
Another way is that she could be a spirit in the Naruto world. She takes the form of a lady named Lemon. She has the power to manipulate nature, like making lemon trees grow instantly. Naruto encounters her in a forest and at first is scared, but then realizes she is good - natured. She helps Naruto gain a new power related to nature chakra by guiding him through a series of trials in the forest.
Well, Goku's super strength and fighting abilities could make him a powerful ally or a formidable foe in the Naruto world. His ki powers might be seen as a strange new form of chakra. He could also teach the Naruto characters some of his unique fighting techniques, like the Kamehameha wave.
In a Naruto fanfic, Alucard could be an ally to some of the characters. His vast knowledge and power could be useful in battles against common enemies. For example, he could help Naruto and his friends in their fight against the Akatsuki. His presence would also add an element of the supernatural to the story. He might teach the ninja new ways of using their chakra or introduce them to different forms of energy manipulation that are similar to his own powers.
A succubus could fit in as a new type of threat. Since the Naruto world already has various threats like the Akatsuki, a succubus with her seductive powers and ability to drain energy could be a unique enemy. For example, she could target the tailed beasts to gain their powerful chakra.
He could fit in as a new type of supernatural being. Since he has god - like powers, he might be seen as a threat at first. But then, as the story progresses, he could form alliances. For example, he could team up with the devils against a common enemy. His abilities like instant teleportation (using techniques like Flying Thunder God) would be very useful in the battles in the Highschool DxD universe.
A swordsman can fit in by being a traveling warrior. He could be like a ronin, wandering from village to village in the Naruto world, offering his sword skills for hire or just looking for worthy opponents. For instance, he might show up at the Chunin Exams to test the skills of the young ninjas.
She could be a new kind of demigod. Since Naruto has such strong chakra and unique abilities in her own universe, in the Percy Jackson universe, this could be seen as a new form of divine power. Maybe she is the daughter of a minor god that has some connection to ninja - like abilities. For example, a god of stealth or speed.
Naruto's abilities would be quite a novelty in the Noblesse universe. His chakra, for instance, is a different form of energy compared to what's in Noblesse. His shadow clone jutsu could be used for various purposes like scouting or overwhelming opponents in battles. His Rasengan could be a powerful attack against Noblesse enemies. And his ability to tap into the Nine - Tails' chakra would make him extremely powerful, perhaps even on a level that would rival some of the strongest Noblesse characters.
Lilith Lemon could be a magical creature in human form. In the Harry Potter universe, there are many such beings. She could have some special powers or abilities that are related to nature. Harry Potter might first be suspicious of her but then realize she is on the side of good and they could team up to protect the wizarding world from a new threat.
In many fanfics, Lucifer's presence can create moral dilemmas for the characters. Percy, who is used to a world of somewhat clear - cut good and evil (in terms of Greek mythology), now has to deal with Lucifer's more nuanced nature. This can lead to interesting internal struggles for Percy as he decides whether to trust or oppose Lucifer.
They would be very powerful. With the Rinnegan, they could potentially overpower many of the regular Force - users. They might be seen as a new type of threat or ally depending on their intentions. For example, if they were on the side of the Jedi, they could use their Rinnegan powers to enhance the Jedi's abilities in combat and strategy.