Well, some fanfictions might focus on the emotional aspect of their relationship. Maybe Naruto helps young Kushina deal with her insecurities or her status as a Jinchuriki. Or perhaps they discover a new power or ability together that was previously unknown. Another plot point could be them changing the course of history in small ways that have big impacts on the future Naruto comes from, all while building a unique bond between them.
One interesting plot point could be exploring how Naruto's presence in the past might change the relationship between him and young Kushina. For example, if he has knowledge of the future, he could try to protect her in different ways.