One common theme is the struggle with pride. Just like in the original, characters often have to battle their own pride to admit their feelings. For example, in many fanfics, Kaguya and Miyuki still go through that internal conflict.
There's also 'Kaguya's New Perspective'. This fanfiction delves deeper into Kaguya's feelings and how she navigates the complex relationship in a more detailed way compared to the original story at times. It's really engaging.
One popular theme is the further exploration of the mind games between Kaguya and Miyuki. Another could be alternate universes where their relationship starts or progresses differently. Also, the development of side characters' stories within the fanfiction universe is quite common.
One common theme is the struggle with pride. In the original series, both Shirogane and Kaguya are too proud to admit their feelings easily, and this is often explored further in fanfiction. For example, they might have internal battles about who should confess first.