In a different take on the fanfiction, there could be a Christmas - themed quest. A demon is causing trouble during the Christmas season, stealing presents and scaring children. Inuyasha and Sango team up to stop this demon. They face many challenges along the way. Inuyasha's brashness sometimes gets them into more trouble, but Sango's calm and calculated approach helps them get out of it. By the end of the quest, they not only save Christmas but also realize that they make a great team and their relationship has become stronger.
Another idea for this fanfiction could be a Christmas celebration in the village. Inuyasha and Sango are both a bit out of place at first. Inuyasha doesn't really understand all the human Christmas traditions, and Sango is still dealing with her past traumas. But as the day goes on, they join in the festivities together. They exchange small gifts, Inuyasha gives Sango a special demon - repelling charm he found, and Sango gives Inuyasha a new sheath for his sword that she made. This simple exchange shows their growing bond.
One possible fanfiction could be about Inuyasha and Sango getting lost in a winter forest on Christmas Eve. They have to rely on each other's skills to find their way back. Inuyasha uses his demon senses, while Sango uses her knowledge of the land. Along the way, they start to see each other in a new light and their friendship deepens.