The main antagonist could be a new, original character created specifically for the fanfiction. For example, a powerful alien sorcerer who has a vendetta against the House of El. He sees Dark Kara as an easier target than Superman, so he focuses his attacks on her. He uses dark magic to try and corrupt her further, making her a weapon against her own family and friends. This sorcerer could also have an army of minions that he sends after Dark Kara at every turn, making her life very difficult.
Lex Luthor could be her main antagonist. He might see Dark Kara as a threat to his own plans of world domination. He could try to manipulate her into doing his bidding or, if that fails, find ways to destroy her. Another option is Brainiac. Brainiac might be interested in studying Dark Kara's new form and powers, and in the process, they could end up in a battle as Kara resists being his experiment.
One potential main antagonist could be Anti - Monitor. He views all of existence in terms of balance, and Dark Kara's presence could disrupt the balance he is trying to maintain. He might try to either convert her back to the 'good' Kara or simply eliminate her. Also, Metallo could be a foe. Since he has kryptonite - based powers, he could be a constant threat to Dark Kara, especially if he teams up with other villains who want to take advantage of her instability.
One idea could be Kara struggling with her dark side as she is exposed to a new form of kryptonite. This kryptonite doesn't just weaken her physically but also affects her mental state, making her more aggressive and less likely to follow her usual moral code. Another plot could involve Dark Kara teaming up with an unexpected villain from another universe, creating chaos across multiple dimensions. And perhaps she starts to question her own identity, whether she is truly the hero she thought she was or if this new dark self is her real nature.
Well, obviously Kara herself is a common character. Then there's often her adoptive family, like the Danvers. Alex Danvers, her sister, is frequently in these fanfictions. They usually have a strong bond that's explored further. Also, Clark Kent might make an appearance. Since he's also from Krypton, he can act as a mentor or a source of information for Kara about their home planet.
One interesting storyline could be Kara adjusting to life on Earth as a young girl. Maybe she's trying to hide her powers while also making friends at school. Another could be her learning about her Kryptonian heritage in a more personal way, like finding a hidden family heirloom that gives her new insights.
In Supergirl fanfiction, a pregnancy involving Winn and Kara can be a very interesting plot device. It could bring out different aspects of their characters. For example, Winn might show his more protective side. However, in the official storylines, there's no such plot, so fanfiction gives writers the freedom to explore this uncharted territory.
Madara Uchiha could be a great antagonist. His powerful Sharingan abilities and his grand plans for the Naruto world would put him at odds with Supergirl. He might see her as a threat to his control and try to manipulate or defeat her. Another option could be Kaguya Otsutsuki. Her god - like powers and desire to reshape the world would clash with Supergirl's values and her attempts to protect the people of the Naruto universe.
Lex Luthor could be a great main antagonist. He could be jealous of their combined power and see them as a threat to his plans of world domination. He might create some high - tech weapons specifically designed to counter their abilities.
A fallen angel who is jealous of Lucifer's newfound friendship with Supergirl and his life on Earth. This fallen angel decides to cause destruction on Earth to get back at Lucifer and also sees Supergirl as a threat to his plans of ruling over the mortals.
It could be an evil ninja from Naruto's world who somehow gains Kryptonian - like powers. He becomes power - hungry and decides to take over both the Naruto world and Supergirl's world. His knowledge of ninja techniques combined with super strength and other abilities would make him a formidable foe.
In fanfiction, when Kara and Lena are married, we can expect a lot of fluff. There will be scenes of them cuddling on the couch, sharing sweet nothings. Their marriage could also mean more united front against villains. They would be each other's strength and support, making them a formidable pair.