Well, in a Naruto - One Piece crossover, characters like Luffy and Naruto would be a must. Zoro could learn some sword - based jutsu from Naruto. And Nami might be interested in the ninja tools Naruto uses for stealth and thievery prevention on the ship.
If it's a Naruto - Bleach crossover, Ichigo and Naruto would be central characters. Rukia could be intrigued by Naruto's ability to control chakra, and Renji might challenge Naruto to a friendly spar to test their different fighting styles.
One potential character could be Harry Potter in a crossover with a magical world. The sirens' powers might be something new and dangerous that he has to face at Hogwarts. He could use his magic skills to try and counter the sirens' alluring voices. Another character could be Katniss Everdeen from The Hunger Games in a post - apocalyptic crossover. The sirens could be part of the Capitol's new weapon, and Katniss would have to find a way to fight against them. And in a crossover with a pirate - themed story, Captain Jack Sparrow could be included. Jack's wily nature might make him both a target and an ally of the sirens.
Well, if it's a crossover with Ultraman, Ultraman could be a great addition. His super strength and light - based powers would contrast well with the Jetman's abilities. They could have a friendly rivalry at first but then team up to face a powerful kaiju that is threatening both their worlds.
You could include a wise old mentor figure, like an old gardener who gives Cinderfella advice. A friendly mouse that helps Cinderfella around the house. And of course, the beautiful princess who Cinderfella falls in love with at the ball.
In a crossover with 'The Last of Us', Joel and Ellie would be great additions. Joel's survival skills and Ellie's immunity could bring new dynamics to the State of Decay world. Also, from 'Left 4 Dead', the four survivors like Bill, Francis, Zoey, and Louis could be included. Their combat skills against zombies could be interesting when combined with the State of Decay gameplay.
Alfred Pennyworth could be included. He could offer his wisdom and support to both heroes. Maybe he even has some insights about the similarities and differences between Gotham and Starling City. Another character could be Felicity Smoak. She could help with the tech - related aspects of the case they are working on together.
Characters from 'Teen Titans' could be great. Their superpowers and the Saturdays' knowledge of the supernatural could lead to some amazing adventures. Imagine Raven's magic and Beast Boy's shape - shifting working together with the Saturdays. Also, characters from 'Gargoyles' could be interesting. The Gargoyles' nocturnal nature and combat skills along with the Saturdays' would create a really unique dynamic in the fanfiction.
Another potential character is Harry Potter. His wizarding skills and knowledge of magic could be a contrast to Naruto's ninja abilities. Imagine Naruto in Hogwarts, learning about magic while also showing off his ninja moves. They could also face a threat together, like a dark wizard who is trying to use both magic and chakra for evil purposes.
Kakashi could also be a great addition. As a mentor figure, he could guide Sakura in understanding and controlling her newfound powers from her rebirth. And then there are the villains. Orochimaru could be interested in Sakura's rebirth if he senses some new and powerful chakra within her. He might try to capture her for his own experiments.
Definitely Zeref. Since he has a connection to the Etherious and also to Natsu in the original Fairy Tail story. Another good character could be Gray Fullbuster. His ice - magic could contrast well with the more chaotic and dark powers of the Etherious. Also, Mavis Vermillion could be included. Her wisdom and powerful magic could play a role in helping Natsu deal with the Etherious side of things.
Joker is a must - include. His chaotic nature would add a lot of drama to any crossover. He could be causing mayhem that draws in characters from other universes or teams. Another one is Catwoman. Her ambiguous relationship with Batman can create interesting dynamics, especially when other characters are involved.