Lex Luthor could be a great antagonist. He could be jealous of their combined power and try to create a device that can split them up or weaken their powers. He might also try to turn the public against them by framing them for crimes they didn't commit.
One idea could be that they team up to face a new and powerful villain from another dimension. This villain has the ability to nullify their powers temporarily, so they have to rely on their wits and non - superhuman skills. Another plot could be that they discover a long - lost Kryptonian artifact that holds the key to saving Earth from a natural disaster, but they have to solve a series of complex puzzles to unlock its power. Or perhaps they meet in a situation where they initially don't recognize each other as Superboy and Supergirl, and there's some humorous misunderstandings before they figure it out.
One aspect to consider is the character development. Superboy might learn valuable lessons about humanity, love, and family values from the Kents. It could also be a story of acceptance, where the Kents welcome Superboy into their family despite his differences. For example, they could help him control his powers better or understand the human world more deeply.
One popular story might be about their adventures together in an alternate universe where they have to team up against a powerful new villain. In this fanfiction, their powers combine in unique ways to defeat the foe.
You can try some well - known fanfiction websites like Archive of Our Own (AO3). It has a large collection of fan - created stories, and you might be able to find some Superboy and Supergirl fanfictions there. Just make sure to use the search function properly.
People write it because they like to explore different relationship dynamics. Superboy and Supergirl are both powerful characters, and seeing them in a romantic relationship can be exciting.
There could be a fanfiction where Superboy and Supergirl are in an alternate universe. In this universe, they are not related as in the traditional stories. They meet at a superhero gathering and the kiss happens spontaneously, leading to a whole new relationship dynamic.
One interesting storyline could be Superboy and Supergirl teaming up to face a new and powerful villain that the Justice League has overlooked. They might discover a hidden threat in Smallville that has ties to Krypton's past.
Often, Superboy is really well - written. Writers can explore his struggle with his identity, being a clone. He has the power of Superman but also his own unique perspective, which makes for great character exploration in fanfiction.
The plot could involve Superboy getting lost in the X - Men's universe. He has to figure out how to get back home while also getting involved in the X - Men's internal conflicts. Maybe there's a power struggle within the X - Men group and Superboy's outsider perspective helps to resolve it. At the same time, he has to deal with the fact that his powers are different from the mutants and some may see him as a threat or an oddity.