One common theme is the exploration of cultural differences. Kagome comes from a modern human world, while Sesshomaru is from a feudal demon world. Their baby would be a unique blend of both cultures. Another theme could be the acceptance of the baby within their respective communities. There might be resistance from some, like Sesshomaru's demon clan who may have strict views on purity. Also, the growth and development of the baby, with its unique powers inherited from both parents, is often a theme. The fanfiction might show how the baby learns to control its powers and find its place in the world.
The baby in the fanfiction could be of a special lineage. It could have powers that Sesshomaru needs to understand and help develop. This would lead to a journey of discovery for him. He may also have to face his own past traumas related to family as he raises the baby, and this could be a way to heal his inner wounds.
The baby could be a half - demon, related to some other important character in the series. Maybe it's the offspring of a character Sesshomaru has some history with.
One great fanfiction where Kagome finds a baby Sesshomaru could be 'Kagome's Unexpected Discovery'. In this fic, Kagome stumbles upon a baby Sesshomaru in a forest glade. The story then delves into how she decides to take care of him, despite their previous enmity. It's a heartwarming tale of an unlikely bond forming between the two.
I'm not sure of a specific titled fanfiction exactly like this, but there could be many on platforms like Archive of Our Own. You can search for tags like 'Sesshomaru', 'Baby Kagome', 'Found Family' to narrow down the results. It's likely that in such a fanfiction, there would be a lot of cultural clashes as Sesshomaru is from a very different world than Kagome, which could make for some interesting plot points.
In many of these fanfictions, Kagome is portrayed as a very protective mother. Given her kind heart and her own experiences in the dangerous world of demons, she would do anything to keep her baby safe. She might use her spiritual powers in new and creative ways to protect the child.
One common theme is family. Despite their often - antagonistic relationship in the original series, fanfictions like to explore how they could have a more positive family - like bond when Inuyasha is a baby. Another theme is redemption. Sesshomaru might be seen as redeeming himself from his past cold - hearted ways by taking care of Inuyasha.
Protection is also a big theme. Sesshomaru is a powerful demon, and in these fanfictions, he often uses his strength to protect Rin and their baby from various threats. Love is another common theme. The love between Sesshomaru and Rin is extended to their baby, and the story might show how this love shapes the baby's life.
There could also be humans who are afraid of the baby's mixed heritage. They might view it as an abomination and try to get rid of it. These humans could be part of a more superstitious group that believes demons and humans should never mix.
One possible plot could be that Sesshomaru, initially annoyed by the presence of the baby, gradually feels a sense of protectiveness. Maybe the baby has some special power or is related to an important prophecy. As he travels with the baby, he has to fend off various enemies who either want to harm the baby or use it for their own gain.