Well, a theme of cultural exchange could be great. Kushina was from the Uzumaki clan and Sasuke is from the Uchiha clan. A fanfic could explore how they share their clan cultures with each other, teaching and learning from one another. Additionally, a theme of self - discovery could be explored. They could both be on a journey to discover more about themselves and in the process, form a deep and meaningful connection that is not based on any inappropriate content.
A theme of family and legacy could be interesting. Since Kushina is Naruto's mother and Sasuke has his own complex family history, they could explore how their respective family legacies shape their identities and how they can support each other in upholding positive aspects of those legacies. Also, a theme of redemption could work well. Sasuke has made mistakes in the past and Kushina, with her kind heart, could be a part of his journey towards redemption.