Well, a big theme is legacy. Hashirama left a huge legacy for the village, and Naruto is constantly trying to live up to that while also making his own mark. There's also the theme of understanding different eras. Fanfics sometimes have Naruto learning about Hashirama's time, which was full of war and strife, and how that contrasts with his own relatively more peaceful (at least initially) era.
Power and its implications are common themes. Fem Hashirama may have different ways of using and controlling power compared to the male version. Her power could be seen as a threat or an asset in the Naruto world. And there's also the theme of adaptation. Both Naruto and Fem Hashirama have to adapt to each other's presence and the changes that come with it in the story, whether it's in combat, in the village, or in their personal relationships.
One common theme is the exploration of different power dynamics. Since Naruto has her own set of powers and Hashirama is known for his great strength, seeing how a female Naruto's powers interact with his can be really interesting. For example, they might combine their jutsu in unique ways.
The theme of unrequited love is also quite popular. It's often depicted that one of them has feelings that the other is initially unaware of. And there's the theme of power balance. How their powers affect their relationship, whether it brings them closer or drives them apart.
One common theme is the complex relationship between them. Given their history in the Naruto series, fanfics often explore how their past rivalries and loyalties translate into a romantic context in a yaoi relationship.
One common theme is the idea of forbidden love. Since Madara and Hashirama are from rival clans, their relationship goes against the norms of their society. Another theme is the power of love to change destinies. Their love could potentially change the fate of the ninja world. For example, it could prevent future wars between the clans.
Reconciliation is also a theme. Given the historical enmity between their clans, a lot of fanfics focus on how fem Madara and Hashirama work towards reconciling their clans through their relationship. They become a symbol of hope for a united future, and this theme adds a lot of depth to the stories.
One common theme is empowerment. Since Hashirama is already a very powerful character in the original Naruto story, when made female, it often emphasizes her strength even more, both in combat and in leadership.
One common theme is their friendship. Despite being on opposite sides at times, many fanfictions explore the deep - seated bond they had. Another theme is redemption. Madara, especially, is often the subject of redemption arcs in fanfiction. Also, the theme of sacrifice for the greater good is seen, as both characters in the original story were willing to make sacrifices for their respective villages.
It could be about many things. Maybe it's about Naruto having some of Hashirama's abilities or traits due to a reincarnation link. It might explore how Naruto's character develops in a new way with this connection, like how he uses Hashirama - like powers in battles or in his relationships with other characters in the Naruto universe.
One common theme is friendship. Despite their differences, the female Hashirama and Madara often form a deep bond that is central to the story. Another theme is power. How they use their power, both individually and together, is often explored.