Perhaps in this fanfiction, some of the Glee characters could have latent superpowers that are awakened when they meet Supergirl. For example, Rachel could discover she has super - speed which helps her get to rehearsals on time and also aids Supergirl in chasing down villains. And the story could explore how they balance their normal lives with these newfound powers and responsibilities.
One idea could be that the Glee club is invited to perform at a big event in National City where Supergirl is also present. They could end up teaming up to save the day when an alien threat targets the event. The Glee kids' unique talents could be used in unexpected ways, like using their voices to disrupt an alien's sonic weapon.
One possible plot could be that the Supergirl characters find themselves in the world of Glee. Maybe Kara has to blend in at McKinley High while also dealing with her superpowers. She could join the glee club and use her super - hearing to help them perfect their harmonies. And she might have to face off against some villains who are after her while trying not to expose her true identity to her new friends in the glee club.
One idea could be a crossover with the Doctor from Doctor Who. The Doctor could land in National City and team up with Supergirl to fight an alien threat that's beyond the capabilities of either of them alone. Maybe it's an enemy that can manipulate time, which is where the Doctor's expertise comes in handy.
Kara and Rachel would be an interesting pairing. Kara with her superpowers and Rachel with her powerful voice. They could learn from each other, like Kara teaching Rachel some self - defense moves and Rachel helping Kara with her stage presence.
One idea could be that a powerful villain threatens both their cities simultaneously. Supergirl and The Flash have to team up, and in the fanfiction, their different powers and personalities could lead to some interesting interactions. For example, Supergirl's strength and The Flash's speed could be combined in unique ways to defeat the villain.
Another concept is that a villain from The Flash's world uses the power of music (inspired by the Glee club's performances) to create chaos. The Flash teams up with the Glee club to reverse the effects of the music - based villainy. The Glee kids could use their knowledge of harmony and rhythm to counter the negative musical energy, while the Flash uses his speed to stop the physical manifestations of the villain's attacks.
One idea could be that the characters from Glee find themselves in Central City during one of The Flash's battles. Maybe they get caught in the crossfire and The Flash has to save them while also dealing with the meta - human threat. For example, Rachel could be singing on the street when a meta - human causes chaos nearby.
Mr. Schuester from Glee could be a main character too. He could be the one who tries to bring the two groups together, seeing the potential in the Glee club and Supergirl working as a team. His role as a mentor in Glee could be extended to Supergirl as she tries to navigate the more complex aspects of human relationships and teamwork in this crossover fanfiction.
One idea could be that a powerful villain creates a time - space rift that pulls both Supergirl and Flash into an alternate dimension where they have to team up to find a way back home. Their powers work differently in this new dimension, and they have to learn to adapt quickly.
Well, a great plot for Supergirl Arrow crossover fanfiction could be centered around a common enemy from another dimension. This enemy has technology that nullifies Supergirl's powers and Arrow's skills, so they have to rely on their wits and each other's unique strengths. For example, Supergirl could use her heat vision in a creative way to expose the enemy's weaknesses while Arrow uses his archery skills to set up distractions. They could also meet some interesting characters from the other dimension during their adventure.
One idea could be that an alien threat emerges which requires the skills of both Arrow's team in Star City and Supergirl in National City. They have to combine their combat styles, with Arrow's more grounded and stealthy approach and Supergirl's super - powered attacks. For example, Arrow could use his trick arrows to create distractions while Supergirl takes on the stronger alien foes directly.