It's hard to say without actually reading a lot of them. But it could be themes like friendship, if they are characters that can form an interesting bond. Maybe adventure if their story takes place in a world full of action.
In 'chrom x orange fanfic', one possible common theme could be overcoming differences. If they are two very different entities, perhaps from different species or backgrounds in the original material, the fanfic might explore how they deal with those differences. Another theme could be loyalty. As their relationship develops, loyalty towards each other could be a central aspect. Additionally, if there is a power dynamic in the original story, the fanfic might play with that, such as one character protecting the other, which could be considered a theme of protection.
Love and sacrifice are common themes. Given their story in the game, their relationship can be explored in terms of how far they would go for each other.
Love and loyalty are very common themes. Their relationship is often depicted as one of deep love and unwavering loyalty. They are committed to each other through thick and thin.
Since I don't have much information on what exactly 'chrom orange' refers to in the context of fanfic, it could be a very niche piece of fan - made literature. It might be that Chrom is a new or relatively unknown character in a particular fandom, and the writer of the fanfic has decided to incorporate the color orange as a symbol, or it could be related to an orange - colored object that is important to Chrom's story in the fanfic. For example, if Chrom is a hero, the orange could be the color of his magic sword or his shield.
Love and friendship are common themes. Their relationship often starts as comradeship and evolves into love. Another theme is loyalty, as they are loyal to each other and their cause. Destiny also plays a role, as in many fics they are seen as destined to be together.
Another common theme is sacrifice. Many fanfics show either Chrom or Sumia being willing to sacrifice for the other. For example, one might be ready to put themselves in danger to save the other from harm. This shows the strength of their bond. Also, themes of leadership and support often come up. Chrom as a leader and Sumia as his partner, how she supports him in his leadership role and how he in turn supports her personal growth.
Well, I haven't read any 'chrom x orange fanfic' myself. But if you join relevant fan communities, like on Reddit or Discord dedicated to the source material where chrom and orange come from, people there might be able to point you to some great ones. They often share and discuss fanfics within the community.
Another theme is leadership. Chrom is a leader, and Robin is his trusted advisor. Fanfictions may delve into how they work together to lead their group. They might face challenges where their leadership styles clash, but then they learn to complement each other and become even stronger leaders as a result.
Love and loyalty are common themes. Since Chrom is a leader, loyalty to him and his cause can be a big part of the story. And of course, the romantic love between Olivia and Chrom. Also, sacrifice is sometimes a theme, like one sacrificing for the other's safety or the safety of the kingdom.
Well, a frequent theme is the emotional connection between them. Writers may depict how Chrom's leadership qualities and Olivia's grace and combat abilities draw them together. Also, there are themes related to loyalty. They are both loyal to their causes, and fanfiction might explore how this loyalty affects their relationship, like if they have to make tough choices between their loyalty to their respective groups and their feelings for each other. This can add a lot of depth to the story.
I haven't heard of any specifically popular 'chrom orange fanfic' stories. It might be a very specific or new concept that hasn't gained widespread popularity yet.