Another option is to include Catwoman. She's a master thief, just like the Assassins in a way. They could work together in a city like Gotham, which has been infiltrated by the Templars. Catwoman could help the Assassin navigate the rooftops and steal important Templar documents.
How about Frodo Baggins? He has the One Ring, which could be a target for the Templars. The Assassin could protect Frodo on his journey to Mordor. The Assassin's skills would be useful in dealing with the various threats along the way, like orcs and Nazgul.
I think Ezio Auditore would be a great addition. He's a beloved character from the Assassin's Creed series. His skills and charisma would make for an interesting interaction with Kassandra or Alexios. They could learn from each other's experiences and combat styles. Another interesting character could be Lara Croft. Her knowledge of archaeology and survival skills would blend well with the world of Assassin's Creed Odyssey. She could join forces with the assassin to explore ancient tombs and uncover hidden secrets.
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.
I think Naruto and Altair would make an interesting pair. Naruto is all about exuberance and his belief in the power of friendship and hard work. Altair, on the other hand, is a more stoic and methodical Assassin. Their different personalities would create great character dynamics. Naruto could learn about Altair's strict code of conduct and Altair could be intrigued by Naruto's ability to keep going no matter how tough the situation is.
Aiden Pearce from Watch Dogs and Ezio Auditore from Assassin's Creed would make an interesting pair. Aiden's hacking skills combined with Ezio's combat and stealth expertise would create a dynamic duo. Ezio could use his knowledge of historical architecture and secret passages, which could be translated into modern - day knowledge with Aiden's hacking to access restricted areas in the city.
Ruby's scythe - wielding skills could be enhanced with Assassin's Creed combat techniques. Her speed could be used to quickly assassinate enemies in close combat. Weiss could use her glyphs in combination with Assassin stealth, for example, creating invisible platforms to move around undetected.
I think Naruto would be the most interesting. His energetic and determined personality would make for great stories in the Assassin's Creed world. He'd be constantly trying to adapt and use his ninja skills in new ways. His ability to make friends and inspire others would also lead to some interesting interactions with the assassins.
I think Sasuke would be quite interesting. His dark past and thirst for power could be paralleled with some of the Assassin's Creed characters' motives. He could learn new ways to use his skills in the Assassin's Creed world and maybe even find a new purpose.
In a crossover with Devil May Cry, Dante would be a great addition. His cocky and powerful attitude would contrast well with the more brute - force nature of the Splatterhouse protagonist. They could team up to fight some sort of demonic invasion that threatens both of their worlds.
Hermione Granger would make a great main character in this crossover. Her intelligence would be useful in understanding the Assassin's Creed lore and how it relates to magic. Also, Altair Ibn - La'Ahad could be a main character. His wisdom as an Assassin could help the wizards in their fight against dark forces. And Ron Weasley could add some humor to the story while also being a valuable member of the team, using his wizarding skills along with the Assassin techniques.
For the Assassin's Creed side, Ezio Auditore could be a great main character. He is a very well - known and skilled Assassin. For the Ghost Rider part, Johnny Blaze would be an obvious choice. Their different skills and personalities would create an interesting dynamic. Ezio's stealth and combat skills combined with Johnny's supernatural powers would make for an exciting story.