In a crossover with 'Naruto', Naruto himself would be a great addition. His positive and determined attitude could contrast with Tomoe's more reserved nature. And his powerful ninja skills could come in handy when facing some of the demons in the Kamisama Kiss world. Sakura could also be there, and she might bond with Nanami over their strong wills.
For a crossover with 'Sailor Moon', Sailor Moon herself would be a natural choice. Her ability to fight evil and her kind heart would make her fit right in. She could team up with Nanami and Tomoe to fight against a new threat that combines the dark forces from both the Sailor Moon and Kamisama Kiss universes. The other Sailor Scouts could also be involved, each bringing their own unique powers.
If it was a crossover with 'Fullmetal Alchemist', Edward Elric would be an interesting character. His knowledge of alchemy and his search for the Philosopher's Stone could somehow tie into the magic in the Kamisama Kiss world. Alphonse could also be included, and the gentle giant could form an unlikely friendship with some of the yokai in Kamisama Kiss.
A crossover with 'Fruits Basket' could also be cool. Nanami could end up in the Sohma family's world and befriend Tohru. Tomoe might be a bit jealous at first, but then he gets to know the Sohmas and they all work together. Maybe there's a curse involved that Nanami and Tomoe try to help break with the Sohmas' unique situation.
One great Kamisama Kiss Mature fanfiction could be the one that focuses on the deeper emotional bond between Nanami and Tomoe as they face more complex challenges in their relationship. It might explore how they grow and change in a more mature way compared to the original story.
If it's a crossover with the Legend of Korra, Korra herself would be an excellent addition. She is a powerful bender and has a lot of experience in fighting evil. She and the Trollhunter could learn from each other's fighting styles. Asami could also be included, providing her engineering skills to help the Trollhunter with any technological needs in their battles against the common foes.
For a crossover with a post - apocalyptic world, characters like Mad Max could be involved. He might see the homunculi as either allies or threats in the harsh new world. In a crossover with a cyberpunk world, characters like Neo from 'The Matrix' could be interesting. The homunculi could be part of the complex digital and physical world that Neo navigates, and he could help them understand the true nature of their existence in this new, high - tech world.
Hermione Granger from 'Harry Potter' could be an interesting addition. She could use her magic knowledge to study the mummy. Maybe she could find a spell to control or at least understand the mummy better. It would be a really unique combination of magic and ancient Egyptian mystery.
In a crossover with 'Fullmetal Alchemist', Edward Elric could be an interesting addition. His scientific knowledge and his search for the Philosopher's Stone could interact with the strange alchemy - like aspects of Rea's resurrection in Sankarea. He might be both fascinated and appalled by what has happened to Rea.
If it was a crossover with The Evil Within, Sebastian Castellanos would be a great character to include. His detective skills and his ability to handle the nightmarish creatures in his own game could be useful in the Outlast 2 world. He could team up with the Outlast 2 character to solve the mysteries behind the horror. Also, Pyramid Head from Silent Hill could be a terrifying addition. His presence alone would heighten the sense of dread in the Outlast 2 crossover story.
Batman would be a great character to include. His no - kill rule contrasts with Deadshot's assassin nature. Their interactions would be full of tension and moral debates. Harley Quinn could also be included. Her chaotic personality could add an element of unpredictability to the story. She might team up with Deadshot for a mission or be a distraction during one of his hits.
If it's a Moana and Hercules crossover, Hercules would be a great addition. His super strength could help Moana in situations where physical power is needed, like moving large obstacles in her path. Also, Philoctetes could be there to train Moana in some combat skills, adding an interesting dynamic to the story.
Definitely include Bill Cipher from Gravity Falls as he is the main villain in Weirdmageddon. Dipper and Mabel Pines are also essential as they are the main protagonists in the Gravity Falls story. If it's a crossover with the Over the Garden Wall universe, Wirt and Greg would be interesting additions. Wirt's cautious and somewhat gloomy nature could contrast with the more adventurous Gravity Falls characters, while Greg's innocence could add a different dynamic.