Beast himself is a great character to focus on. His intelligence and his struggle with his bestial form can lead to a lot of interesting storylines. For example, a story could explore his scientific research to find a cure for his mutation while also dealing with the moral implications of changing who he is.
Another good character is Wolverine. In a Beast X Men fanfiction, Wolverine's no - nonsense attitude could contrast with Beast's more intellectual approach. Their relationship could be explored, like when they disagree on a mission strategy but have to work together to overcome an enemy.
Wolverine is always a great choice. His tough - guy attitude and healing factor make him adaptable in any crossover situation. He could hold his own against powerful characters from other universes. Another good one is Storm. Her control over the weather can be a game - changer in battles. In a crossover, she could use her powers to combat foes that others might struggle with.
Loid is a great character to focus on. His double life as a spy and a father offers a lot of interesting story possibilities. You can explore his internal conflicts and how he manages to balance his dangerous job with his new family life.
The main family members can be great to focus on. Their relationships, power struggles, and dreams offer a lot of material. For example, the patriarch of the family who has to make tough decisions to keep the ranch going. Another good character could be the strong - willed matriarch who holds the family together. Also, the young and rebellious heir who wants to do things his own way.
From the Regeneration side, if there is a character with some sort of energy - based powers, they could be a good focus. Paired with Robin from Young Justice, they could create some really interesting strategies in battles. Another option could be a character who has a strong moral code from Regeneration, and when put together with Aqualad from Young Justice, they could have some great discussions about leadership and responsibility.
Cannonball is another great character. His power to fly while encased in a protective force field is cool. In fanfiction, you could focus on his leadership potential within the New Mutants. He's often seen as a reliable and strong - willed character, and exploring his relationships with other mutants could be really interesting.
Sersi is a great character to focus on. She has a strong connection to humanity and her powers are quite interesting. You could explore her relationships with humans in more detail. Another good one is Ikaris. His loyalty and the internal conflict he might face are good material for a fanfiction. Then there's Kingo, with his love for movies and his unique charm among the Eternals.
All of the members of 4 town are great to focus on in different ways. Tae Young, for example, might be the subject of a fanfiction where his amazing dancing skills are the center of the story. Maybe he gets an offer to be in a big dance competition outside of the band, and he has to decide whether to take it or stay loyal to 4 town. Each member brings their own unique traits that can be explored in fanfiction.
Obviously, the beast and the belle are the main characters that are often focused on. Their relationship is the core of the story, so exploring their personalities, growth, and love for each other is always popular.
Charlie is a very popular character to focus on. His wild lifestyle and womanizing ways offer a lot of material for fanfiction. Writers can explore his past relationships more deeply or create new ones. Alan is also popular as his more nerdy and often unlucky in love character can be developed in different ways, like finding true love or having a mid - life crisis.
The friends of the main couple can also be great to focus on. They can add humor, support, or even create some conflict. For example, a friend who is always teasing the couple about their relationship can be a fun addition to the fanfiction.