If we think about it, Connie could also be a hero in 'Beach City Gets Invaded' fanfiction. She has been trained in sword fighting and has a lot of courage. She could team up with Steven and the other Crystal Gems. Pearl, with her grace and strategic mind, is another likely hero. She can come up with battle plans and use her spear - fighting skills effectively against the invaders.
Lion could play an important role too. He can teleport, which could be useful for getting the characters out of dangerous situations or for surprise attacks on the invaders. Even some of the human characters like Sadie and Lars might step up. They could use their knowledge of the city to help in the defense. For example, they know all the secret passages and hiding places in Beach City that could be used against the invaders.
Steven would be a very likely hero. His unique abilities from his gem heritage would be a great asset in fighting off the invaders. Garnet, with her future vision and strength, would also be a top contender. She could see the moves of the invaders before they make them. And then there's Amethyst, who is always ready for a fight and can use her shape - shifting to her advantage.
One common plot point could be the different types of invaders. For example, they might be aliens with strange powers. The citizens of Beach City would then have to figure out how to defend themselves. Another plot point could be the role of the main characters. Steven Universe, if it's related to that universe, might use his gem powers to fight back. Also, there could be a part where the invaders have a specific goal like stealing the beach city's natural resources or some special power source that exists there.
Issei could be a likely hero in such fanfiction. Even though he has a devil side, his human side might make him conflicted about the actions of some devils. He could turn against the devils who are causing harm and use his powers to bash them in order to protect his friends and the school.
Naruto himself is very likely to be involved. He's the main character of his series and his unique abilities like the Rasengan and shadow clones would make for great interactions. Sasuke is also a top candidate. His Sharingan and his dark past could create interesting storylines when combined with hero concepts. Sakura, with her medical ninjutsu and strength, could also be involved, perhaps teaming up with a hero who has a similar healing - based power.
In a fanfiction, it could be a shadowy organization. They might have heard rumors about Raven's unique powers and decided to take her. This organization could be researching ways to gain superhuman abilities and believe Raven holds the key. Or it could be a deranged fan of Raven's powers. He might be obsessed with her and think that by kidnapping her, he can have her all to himself. Additionally, a vengeful spirit from Raven's past could be the kidnapper, seeking revenge for something she did long ago.
Enemies from her past, such as old villains she's fought against, would be likely kidnappers. They might hold a grudge for her previous defeats and want to use her for revenge or some dark purpose.
A new, unknown villain. Maybe a brilliant scientist who turns to the dark side after a personal tragedy. This villain could create a weapon specifically designed to counter Batman's abilities, like a suit that can absorb all of Batman's punches and then redirect the force back at him, ultimately killing him.
One of the most likely possessing entities could be Bill Cipher. He's a powerful and malevolent being in the Gravity Falls universe, and he has shown an interest in Dipper before. Another possibility is an ancient spirit or demon from the forest around Gravity Falls. The forest is full of mysteries, and it's not hard to imagine some dark entity lurking there that decides to possess Dipper.
Leo Valdez is another character who could be a star. His ability to create and manipulate fire can add a lot of excitement to party games. He could be involved in a game where he has to use his fire to light up targets or keep a magical fire - based object alive for a certain amount of time.
It could be an old enemy seeking revenge. Maybe a powerful Titan who escaped Tartarus and blames Percy for his previous defeat. This Titan could have been plotting in secret, waiting for the perfect moment to strike and finally end Percy's life.
Bakugo would be a very likely candidate. He and Kirishima are good friends, and Bakugo has a strong sense of loyalty (even if he doesn't always show it in the most obvious way). His powerful quirk and his determination would drive him to go after Kirishima and bring him back.