For a crossover with the Lord of the Rings, Aragorn could be the main character. As a skilled warrior, he would be one of the few who could stand against a Deathclaw. His leadership skills could also come into play as he tries to protect his people from this new and terrifying threat.
If it's a crossover with the DC universe, Batman could be the main character. He would use his detective skills and arsenal of gadgets to face the Deathclaw. His dark and brooding nature would make for an interesting contrast against the brute force of the Deathclaw.
Perhaps some of the more mysterious characters from the series could take the lead. For example, Isabel Kabra. Her devious nature could add a whole new layer of intrigue to the crossover fanfiction. She could be working with or against the other characters in unexpected ways, creating a lot of tension and interesting plot twists.
A group of scientists who were involved in creating the Indoraptor in the first place could be main characters. They might have to face the consequences of their creation when it crosses over into another world or situation. They would be trying to either control it or destroy it.
Definitely, Cortana could be a main character in a 'halo anime crossover fanfiction'. If she were in the world of Code Geass, she could help Lelouch with his strategic plans, using her vast knowledge and intelligence. Another option is the Elites from Halo. They could be the main characters in a crossover with My Hero Academia. Their physical strength and warrior culture could be contrasted with the superheroes in that anime, and they could have some interesting interactions with the students at UA High.
Obviously, Naruto would be one of the main characters. He's the protagonist of the Naruto series and would be the main link to the ninja world in this crossover. Another could be a young Kitsune, maybe one that's just coming of age in the Kitsune world. Their paths cross, and they have to learn to work together.
If the 'accel world crossover fanfiction' is with a high - tech world, Ash Roller could be a main character. His unique style and skills in the 'accel world' would translate well in a high - tech crossover. He could be the one to introduce the 'accel world' concepts to the new world. And if it's a crossover with a more mysterious world, Sky Raker could be a main character with her wisdom and mysterious past.
Well, if it's a crossover with the Transformers universe, Optimus Prime could be a main character dealing with the battle droids. He could try to teach them about honor in combat, which is very different from their programmed aggression.
It could be a brave plumber who is used to dealing with normal toilets but is now faced with the skibidi toilets. He would use his plumbing skills in creative ways to combat them.
Lightning McQueen from the cars world could be one of the main characters. He could be curious about the planes and get involved in an adventure with them. And from the planes side, maybe Dusty Crophopper. They could form an unlikely duo and face challenges together.
Anakin Skywalker could be one of the main characters. Since he has such a complex journey on the Jedis path, if he were to cross over into another time or alternate Jedis path, it would be very interesting. He could interact with other Jedis, like Luke Skywalker, and they could compare their experiences and understanding of the force.
Obviously Naruto himself would be a main character. He could be joined by Sasuke. In a crossover with 'One Piece', Luffy could also be a main character in the fanfiction, as he is the captain in 'One Piece' and would bring his own unique charm and abilities.