One possible storyline could be them teaming up against a new, powerful alien threat that neither the UNSC nor the Covenant can face alone. They have to put aside their differences and combine their unique skills and knowledge. Master Chief's combat expertise and the Female Arbiter's understanding of Covenant politics and technology would make for an exciting partnership.
For Master Chief, his character could be developed by showing his reaction to working with a former enemy like the Female Arbiter. We could see his internal struggle as he has to trust someone from the Covenant. For the Female Arbiter, her character can be developed by exploring her view of humans through her interactions with Master Chief.
One interesting storyline could be them teaming up against a new, powerful alien threat that neither the UNSC nor the Covenant remnants can face alone. They start off distrustful but gradually learn to respect each other's skills and cultures.
Their relationship can be developed through shared battles. For example, when facing a common enemy, they start to rely on each other's unique abilities. Master Chief's superhuman combat skills and Arbiter's leadership within the Covenant structure can complement each other.
The story could be set in a post - war scenario where the male human and the female arbiter are part of a reconstruction effort. They have to work together to rebuild a planet that was destroyed during the war. Their relationship starts off as a professional one but as they face difficulties like resource shortages and hostile survivors, they grow closer. They learn to rely on each other's skills, with the male human's engineering skills and the female arbiter's leadership abilities complementing each other.
Perhaps the story could revolve around a diplomatic mission. The male human is sent as an envoy to the Arbiter's people. The female arbiter is assigned to protect and guide him. Along the way, they have to navigate through political intrigue within her own people. The male human, with his different perspective, helps the female arbiter see new ways to solve the internal issues. And in the process, they develop a deep bond that transcends their species differences.
A lot of fanfictions also focus on cultural exchange. The Master Chief is from a human military background, while the Arbiter is from the Covenant. Their interactions can lead to interesting cultural exchanges. For example, they might learn about each other's beliefs, values, or military strategies. Some fanfictions may also delve into a more complex relationship where they have to overcome initial distrust or misunderstandings due to their species' past conflicts.
A plot could revolve around a mystery that involves both of their species. The Female Master Chief and the Arbiter start investigating it separately, but their paths cross. As they dig deeper, they realize that their fates are intertwined, and they must rely on each other not only to solve the mystery but also to survive the dangerous forces that are trying to stop them.
Well, one possible plot could be a story of alliance. Maybe they team up to face a common enemy that threatens both of their species. They start off distrustful of each other but as they fight side by side, they begin to respect and understand one another.
Some Halo fan communities on Discord may also have fanfiction channels. Members often share their own works or know of good stories. You can join a few relevant Discord servers and explore the fanfic sections there.