Stiles would also be a great choice. His intelligence and quick - thinking in Teen Wolf could be a valuable asset in the Fate world. He might be the one to figure out how the different magic systems work together. Also, his sarcastic and humorous nature could add a light - hearted touch to what could be a very complex and dark crossover story, while also getting into some deep and dangerous situations.
Scott would be really interesting. His role as an alpha werewolf could lead to some great storylines in the Fate universe. He could be seen as a powerful being there, and his sense of leadership and loyalty would be put to the test. He might have to make difficult decisions about which side to take in Fate's complex power struggles.
Ruby would be really interesting. Her innocent and positive attitude would contrast greatly with the dark and desolate Fallout world. She'd be like a ray of hope in the wasteland. Her scythe - fighting skills would also be unique and useful against the various threats in Fallout. She might try to befriend some of the mutants or ghouls, seeing them not as just monsters but as living beings who need help.
Sinister characters like Adam Taurus from RWBY could also be interesting in a Bloodborne crossover. His already dark and violent nature could be enhanced in the Bloodborne setting. He could form alliances with some of the more sinister forces in Bloodborne, or perhaps he becomes a sort of anti - hero, fighting against the greater evils in the Bloodborne world while still being a morally ambiguous figure.
I think a pairing between Shiba Tatsuya from Mahouka and Archer from Fate would be really interesting. Tatsuya is a very powerful and intelligent magician in his own right, and Archer has a wealth of combat experience and a complex personality. They could have some great philosophical debates about the nature of magic and the purpose of their powers.
One idea could be that the characters from Teen Wolf end up in the Fate universe and have to deal with the complex magic system there. For example, Scott could try to use his werewolf abilities to control the powerful magical forces in Fate. Another could be a love story where a character from Teen Wolf falls in love with a Fate character, like Stiles falling for a Fate mage. Maybe the Teen Wolf pack has to fight against a common enemy in the Fate world, something that combines the bestial nature of the werewolves with the magical threats of Fate.
I think Leina would be a great choice. She has a strong sense of justice and is always looking to improve herself. In a crossover, she could be the one to bridge the gap between different groups or worlds. For instance, if crossed over with a space - faring civilization, she might be the first to try and understand their technology and culture, while also showing them the values of her own world.
Luna Lovegood would be an unexpected but fascinating choice. Her unique perspective and belief in all kinds of magical creatures could make her very open - minded about the vampires and werewolves in the Twilight world. She might see them as just another type of magical being and try to find common ground, which could lead to some really interesting interactions and storylines.
Definitely Alex Mercer. His powers are so unique compared to the Justice League members. He can shapeshift into anyone, has superhuman strength, and can regenerate from almost any injury. This would create a lot of interesting interactions with the League. For example, he could transform into Superman to test the limits of his own power against the real Superman, which would lead to a really cool showdown.
Raphael could be really interesting. In the 2003 version, he is hot - tempered and tough. In the 2012 version, he also has a strong personality. Seeing how he reacts to his counterpart and how they might either clash or bond could be a great focus. Maybe they have a friendly rivalry as they compare their fighting skills.
Ross could be really interesting. Since he's an archaeologist, in a crossover with a show like 'Indiana Jones' (in a fanfiction sense), he could go on an adventure with Indiana Jones. Ross would bring his academic knowledge while Indy would have the practical experience. Their interactions would be filled with Ross trying to over - analyze everything and Indy just wanting to get the job done.