One common theme is family. Since she is Kagome's sister, the story often explores their sisterly bond and how it affects the plot. For example, the sister might be very protective of Kagome.
Well, a really engaging kagome's little sister fanfiction could be the one where she gets transported to a different era than Kagome. In this new setting, she has to adapt and find her own place. She might meet new characters and face different challenges that are unique to her, which makes for a great read as it gives a fresh perspective on the whole Inuyasha - related world.
One common theme is friendship development. Since they start off not really understanding each other well, many fanfictions focus on how they gradually become friends. For example, they might share a common goal which forces them to work together and in the process, they learn about each other's strengths and weaknesses.
One popular little Inuyasha and Kagome fanfiction is 'A New Adventure'. It takes their relationship in a different direction, with them exploring a new realm together. It has really cute moments between them as they face new challenges side by side.
One possibility for Sansa's adopted sister in fanfiction is that she could be a spy. Maybe she was adopted by the Starks under false pretenses, but actually sent by an enemy. However, as she grows up with Sansa, she starts to have a change of heart. She could then use her position to feed false information back to her original masters, all while protecting Sansa and the Stark family from harm.
A common plotline is the sister having a special power or ability that aids Naruto. For example, she might have a healing ability that helps Naruto recover from his battles faster. Another is the sister being involved in Naruto's search for Sasuke. She could be the one who tries to convince Sasuke to come back to the Leaf Village when Naruto fails at first.
Bankotsu and Kagome fanfiction is interesting because it combines two very different characters. Bankotsu is often seen as a rough and tough warrior, while Kagome is more gentle and spiritual. A fanfiction could play on this contrast, for example, having Kagome's kindness soften Bankotsu's hard edges.
One interesting Naruto adopted sister fanfiction could be 'The Adopted Uchiha'. In this story, the sister, who is adopted into the Uzumaki family, has some hidden Uchiha lineage. It shows her journey of discovering her powers and fitting into the Naruto universe. Another great one is 'Sister of the Jinchuriki' where she plays a crucial role in Naruto's life, helping him control the Nine - Tails' power at times.
Kagome would probably be shocked at first. Then she would start yelling at Inuyasha, demanding to know why he's doing this. She's not the type to just sit there and take it.
Inuyasha would likely become more protective. He's always been protective of Kagome, but with her pregnant, it would be on a whole new level. He might not let her go on dangerous missions as easily. Kagome, on the other hand, would probably get frustrated with his over - protectiveness at times, but also appreciate his concern deep down.