There's a fanfiction titled 'Meruem's Discovery'. In this, it delves into how Meruem starts to understand deeper emotions through his time with Komugi. It has detailed descriptions of their encounters and how Meruem's view of the world shifts because of her. Well, it's really interesting as it also shows Komugi's unwavering nature in the face of this powerful being. It's like she has a power of her own, not in strength but in spirit, which gradually influences Meruem.
A fanfiction could focus on an alternate universe where Meruem doesn't die and he and Komugi explore different parts of the Hunter x Hunter world together. They could encounter new species and cultures, all while Meruem continues to learn from Komugi's innocence and wisdom.
Often, it depicts their relationship as a very unique one. It shows Meruem being initially dismissive but slowly growing to respect and care for Komugi.
You can develop their relationship by showing more of their shared interests. For instance, Meruem's growing fascination with Gungi as Komugi teaches him. This shared activity can be the basis for deeper emotional connections.
The Hunter x Hunter manga is unfortunately not complete. The author has faced various issues that have led to intermittent breaks and an unresolved storyline.