Perhaps a fanfiction where Supergirl arrives in Smallville during a time when Clark is struggling with his identity. She could be a source of inspiration for him, showing him the full potential of his powers. They could also have a friendly competition to see who can save more people in Smallville, which would be a fun and exciting plot.
A really cool storyline might be a crossover during a crisis event. Supergirl and Clark Kent from Smallville could be joined by other heroes from both of their universes. They would have to put aside their differences and work together to save both Smallville and Supergirl's city. This would involve a lot of character development as they learn about each other's worlds and form new relationships.
Another interesting idea is a mind - control situation. A powerful telepath takes control of some of the heroes and villains in their respective cities. Flash, Arrow, and Supergirl have to find a way to break the mind - control. They start by finding the source of the telepath's power. Along the way, they discover that the telepath has a tragic past that led to this misuse of power. They try to reason with the telepath while also fighting off the controlled allies and villains.
Another theme is the power struggle. Smallville often focused on Clark's journey to control his powers, and Supergirl has her own power - related challenges. In crossovers, they might face a common enemy where their combined powers are needed, and this creates a power struggle dynamic as they figure out how to work together effectively.
One great storyline could be a villain that is causing chaos in both Arrow's Star City and Supergirl's National City. The two heroes team up, with Arrow using his combat skills and Supergirl using her superpowers to take on the baddie. They might initially have some differences in their approaches, but over time, they learn from each other and form a strong bond.
One great Smallville Supergirl crossover fanfiction is 'Smallville's Kryptonian Visitor'. It has an interesting plot where Supergirl lands in Smallville and has to navigate the already complex world of Clark Kent's hometown. The characters are well - written and it stays true to the essence of both shows.
There are fanfictions where the advanced technology of Stargate is introduced into the Smallville universe. For instance, imagine the Langarans sharing their medical knowledge with the Smallville doctors. This could lead to some interesting character development as the Smallville characters have to adapt to new ways of thinking. Also, a crossover could explore how the alien races from Stargate would react to a being like Clark Kent with his superhuman abilities.
There are fanfictions that focus on the crossover during a time - travel event. For instance, Clark Kent accidentally travels to the 'Arrow' universe. There, he has to adapt to the darker and more urban setting. He could end up sharing his powers and knowledge with Oliver's team, and together they face a powerful meta - human that neither could defeat alone. This type of fanfiction often explores how different the two universes are in terms of their heroes' methods and the overall tone of their worlds.
Another great storyline could involve a cross - dimensional rift that brings characters from Arrow's world into Supergirl's and vice versa. This would create a lot of chaos as the characters have to adapt to the different rules and environments. For example, Oliver might struggle with the presence of aliens in Supergirl's world, while Kara has to deal with the more gritty and crime - ridden Star City. It could also lead to some interesting character pair - ups and new friendships or rivalries.
A really cool story could be set in an alternate universe where Supergirl and Batman are the last two heroes left. They have to rely on each other completely and in the process, they discover new aspects of their powers and themselves. Their relationship could evolve from just being allies to something more complex and deep, with Supergirl teaching Batman to have more faith and Batman teaching Supergirl more about strategy.
One great storyline could be them teaming up to fight a common enemy like a powerful meta - human who is causing chaos in both of their cities. They could start off not trusting each other fully due to their different methods but then grow to respect one another as they see the value in each other's skills.
One popular storyline is when Flash and Supergirl team up to fight a common enemy from another dimension. This allows for a lot of action and showcases their different powers working together.