Ron Weasley could be another main character. His loyalty and his ability to bring some humor to tense situations would make him an important part of the group. Also, his skills in wizard chess might translate to strategic thinking in other areas.
Hermione Granger would also be a main character. Her intelligence and vast knowledge of magic would be invaluable in any situation the heroes face. She could come up with strategies and solutions.
Obviously, Harry Potter himself would be a main character. He has the most experience in fighting dark wizards.
One interesting plot could be the heroes from different Hogwarts houses uniting against a new dark wizard threat that is more powerful than Voldemort. They might have to combine their unique magical skills and knowledge.
Wonder Woman is also very popular. Her strength, grace, and the message she represents attract a lot of fans. Writers can explore different aspects of her character, like her relationship with other heroes or her adventures in new storylines.
Perhaps the heroes assemble to save a powerful magical artifact that is being targeted by a group of dark forces. Each character could bring their own knowledge and abilities to the quest. For example, Hermione's vast knowledge of spells and potions would be crucial, while Harry's courage and experience in facing dark wizards would lead the group. They might also encounter new magical creatures along the way that they have to either befriend or fight.
Obviously, Harry Potter himself is a likely hero. His experience in fighting Voldemort has given him the skills and courage to face new apocalyptic threats. Hermione Granger is also a strong candidate. Her intelligence and vast knowledge of magic would be invaluable in trying to solve the problems causing the apocalypse. Ron Weasley, with his loyalty and ability to think on his feet, would be right there beside them, fighting against whatever doom is approaching.
Another interesting aspect could be the origin stories of the villains in heroes assemble fanfic. A villain might have a tragic past that led them to their evil ways, and the heroes have to figure out how to stop them without just using brute force.
Thor is quite popular in heroes assemble fanfiction. His god - like powers and his fish - out - of - water moments on Earth can create really interesting plots. And of course, Black Widow, with her mysterious past and combat skills, is also a beloved character in these fanfictions.
It could be a Muggle - born protega. This character would face unique challenges as they try to protect the wizarding world while also dealing with their own identity issues. They might have friends among the Hogwarts students, and together they form a group to counter a new evil force that targets both Muggles and wizards.
Obviously, Harry Potter would be one of the main characters. He's the hero of the original series and would likely play a central role in any fanfic about the basilisk. Hermione Granger would probably also be there, using her intelligence to help solve the problems related to the basilisk.
I don't know for sure as I haven't read this particular fanfic. But if it follows the general 'Harry Potter' lore, it could be Harry, Hermione, and Ron. They are often central characters in many fanfics and could be involved in whatever 'purge' is taking place in this story.