Finn might also be a good choice for the main character. He has his own insecurities and has made some not - so - great decisions in the show. Maybe he does something related to his relationship with Quinn or Rachel that he thinks they'll hold against him.
It could be Rachel. She's very ambitious and often steps on toes to get what she wants. So a story where she does something that she's afraid others will hold against her, like stealing a solo opportunity from someone else, would be interesting.
Another possible plotline could be a new member joining the Glee club who has a bit of a dark past. They're worried that the others will find out and hold it against them. So the story could be about their journey of trying to fit in while hiding this secret, and how eventually they either have to come clean or their secret gets revealed in a very unexpected way, causing a stir in the whole Glee club.
I think Rachel Berry would be a great main character. She was always so driven and her journey from a small - town girl with big dreams to a potential Broadway star was so interesting. In a reunion, she could be the one to bring everyone together and also face her own insecurities about whether she has really achieved all she wanted.
Rachel Berry could be a great main character. Given her ambitious nature, being struck by lightning could add an extra layer of drama to her journey of achieving stardom through the glee club.
Sasuke could also be the main character. Maybe in this fanfiction, he is trying to redeem himself against all odds. He has to face the consequences of his past actions and at the same time, fight against new enemies that target him because of his past with Orochimaru and the Akatsuki.
Finn Hudson could also be a great main character. He might see himself as a shooting star in the context of his football and glee club life. He has to balance these two worlds and his journey could be like that of a shooting star, trying to shine in both areas and make a mark in his school and community.
I think Loki would be really interesting. He's always the trickster in the Avengers world. In a crossover with Glee, he could use his magic to create some wild and wacky musical illusions. He might even try to take over the Glee club and turn it into his own little kingdom of chaos.
The children of Rachel and Finn could be main characters. Maybe their son or daughter inherits Rachel's amazing singing talent and Finn's charm. They could struggle with living up to their parents' legacy in the glee club.
Barry Allen from The Flash and Rachel Berry from Glee could be main characters. Barry's speed could be used in interesting ways, like helping Rachel get to auditions on time or saving her from some danger in the city. They could form an unlikely friendship and be at the center of the story.
Finn Hudson could also be a main character. He could be the one who has some initial reservations about the whole royal welcome thing but then slowly gets into the spirit of it as he sees how much it means to the other members of the Glee club.
Maybe a disgruntled former member of the Glee club who feels wronged. This person has a vendetta and uses their knowledge of the club's weaknesses. They lurk on the 'highway to hell' and try to sabotage the club's journey, for example by stealing their sheet music or spreading rumors among the members to cause internal strife.