In the manga, the duration of Gojo's seal is not clearly defined. It's left somewhat open-ended, allowing readers to interpret it based on the context and events that follow.
Friendship - turned - more - than - friends is also quite common. They start as colleagues or friends in the 'Jujutsu Kaisen' universe, and in fanfics, it often evolves into a deeper relationship, whether it's romantic or a very close platonic bond with strong feelings. Another theme could be their shared experiences in battles. How they rely on each other in dangerous situations and how that affects their relationship off the battlefield.
One popular theme is the exploration of their contrasting personalities. Gojo is more carefree and powerful, while Nanami is serious and methodical. Fanfics often play with how they interact and complement each other because of these differences.