One idea could be a story where Goku and Kefla have to team up to face a new, powerful enemy that neither could defeat alone. They start off not trusting each other much, but as they fight side by side, they learn to respect one another's strength and fighting styles.
One idea could be that the OC is a powerful warrior from a different dimension. They meet Kefla during a universal tournament. Their powers are so different yet complementary that they start to train together, and gradually a deep bond forms between them as they learn from each other's fighting styles.
A possible plot could involve Kefla being sent to the Naruto world as part of a test by the Kais. In this world, she is intrigued by Naruto's determination. Naruto, on the other hand, is amazed by her power. They train together and learn from each other's fighting styles. This leads to a strong bond between them, and they end up protecting the Hidden Leaf Village from a powerful new threat.
Sure, there are quite a few. These fanfictions often take the strong and unique personalities of Goku and Kefla and weave them into a romantic narrative. Some might be more action - packed with their love story developing in the midst of battles and challenges, while others could be more about the quiet, tender moments between them. It really depends on the creativity of the author.
A Goku crossover with the Pokémon world could be really fun. Goku could try to catch Pokémon using his speed and strength. He might also team up with Ash to battle powerful Pokémon trainers. Or how about a crossover with the Digimon world? Goku could enter the Digital World and help the Digimon in their battles against evil Digimon using his energy attacks.
One idea could be a crossover where Goku somehow ends up in the RWBY universe. He could train the RWBY characters, sharing his martial arts knowledge and helping them improve their combat skills. For example, he could teach Ruby how to better control her speed and use her scythe more effectively in combat.
One idea could be a fanfiction where Kefla has to team up with Goku and Vegeta to face a new, extremely powerful enemy that is threatening the entire universe. This would explore how she would interact with them on a more equal footing outside of a fight - to - the - death scenario. Another idea is a story about Kefla's life on her home planet before coming to Earth and her training there.
One idea could be that Goku and Goku Black are forced to team up against an even greater threat from another universe. They start to understand each other's fighting styles during the battle and gradually develop a grudging respect for one another.
In some versions, the female Goku could be part of a prophecy. The two Gokus meet, and at first, there is some confusion and misunderstanding. But as they get to know each other, they discover that they are key to fulfilling the prophecy. They combine their powers and defeat an ancient evil that was threatening both of their worlds.
You can expect a lot of creativity. Since it's fanfiction, the authors can play around with the characters' personalities and the Dragon Ball universe rules. Maybe Goku shows a more romantic side that we don't often see in the original series, and Kefla could be softened in some ways to fit the love story.
One idea could be Goku becoming more cynical. Maybe after facing a great loss, he loses his usual optimistic view of battles and training. Instead of seeing every fight as a chance to grow, he starts to question the point of it all.