A young and brave explorer could be an interesting addition. This character is drawn to the mystery of the 'ghost' and the 'darkness' and decides to study them up close. Along the way, they face many challenges and also start to develop an almost symbiotic relationship with the two entities, learning things about themselves in the process. The character could bring a fresh perspective and a sense of adventure to the fanfiction.
The main characters from the original story are always interesting to expand on. But also, a local shaman type character could be fascinating. They could have knowledge about the 'ghost' and the 'darkness' and try to use their powers to either stop or control them.
Ash is obviously the most popular character to include. He's the protagonist of Army of Darkness and has a very distinct personality. His one - liners and his badassery with his chainsaw and boomstick make him a great addition to any crossover.
The Sage of Six Paths could be included. As he is the originator of many ninja powers, his connection to Amaterasu could be explored. Maybe he had a hand in creating or containing the power. His presence in the fanfiction would raise the stakes and give a more epic feel. Additionally, Hinata could be in the story. Her love for Naruto could play a role, for example, if Naruto is in danger because of Amaterasu, Hinata could try to save him in her own way.
Sasuke is a great character to include. His cool and brooding personality can create interesting dynamics in crossovers. For example, if he crosses over into the world of Attack on Titan, his skills as a ninja would be put to the test against the Titans. Another interesting character is Kakashi. His laid - back yet highly skilled nature can add humor and action to crossovers. He could be a great addition in a crossover with the world of Inuyasha, sharing his knowledge of combat and jutsu.
A group of thieves could also be included. They might initially try to steal from the Prince of Darkness but end up getting caught in his web of power and secrets. Their street - smart skills and lack of scruples could create some interesting dynamics as they interact with the Prince and other more noble characters.
Darkness is quite an interesting character. Her masochistic tendencies make her unique. She is a crusader but has this odd quirk that sets her apart from typical heroic characters.
I think Shadow could be an interesting addition. His brooding nature and his own set of powers would contrast well with Sonic's more upbeat personality. He could also have his own reasons for getting involved in the Sonic - Dark Gaia conflict. Maybe he has some past connection to Dark Gaia that no one else knows about. Also, Amy. Amy's love for Sonic can drive her to do some really brave things in the face of Dark Gaia's threat, which is always an interesting dynamic.
In Mystreet, Aaron would be an interesting choice. His mysterious past and combat skills could add a lot to the crossover. For Skymedia, a character who is an expert in technology or has some sort of unique power related to their world's mechanics. When these characters meet, there could be some really cool moments. For example, Aaron might be in awe of the technology - based powers of the Skymedia character, while the Skymedia character could be intrigued by Aaron's more traditional combat skills.
One interesting character to include would be Punisher. His extreme methods and lack of regard for the law would be a great foil to Daredevil's more law - abiding nature. Spider - Man is also a good choice. His youth and naivete, along with his sense of responsibility, would create some interesting dynamics. And don't forget about Luke Cage. His strength and his loyalty to his community could be a great asset in the story. He could side with Daredevil or be caught in the middle of the conflict between different factions.
Definitely Connor and Haytham. But also consider including the character of Aveline de Grandpré from Assassin's Creed III: Liberation. She could cross paths with Connor in a fanfiction, perhaps they team up to take on a common enemy. Another character to think about is Edward Kenway, Connor's grandfather. His adventures and skills could be passed down through the family and influence Connor in some way. And don't forget about the civilians in the game. Using a civilian's perspective to view Connor's actions and the conflict between the Assassins and Templars can be a very fresh approach.
Ben Tennyson is definitely an interesting character to include. His ability to transform into different aliens gives a lot of possibilities for the story. Finn the Human is also great. His adventurous spirit and his sword - fighting skills can add action to the fanfiction. Jake the Dog can be really fun too, with his shape - shifting abilities. And of course, Princess Bubblegum, her intelligence and leadership can bring a different dynamic to the story.