One common theme is adventure. Given their background in the Fire Emblem series, many fanfics will have them go on quests together, facing various challenges and enemies. Another theme is romance, of course. Writers often explore the relationship between Ike and Female Robin, how they fall in love, and how they deal with the difficulties that come with it.
You can try fanfiction websites like Archive of Our Own (AO3). There are a large number of fanfics about Ike and Female Robin on it. The search function there is quite powerful, allowing you to filter by various criteria such as rating, length, and genre to find the ones that suit your taste.
One common theme is friendship. Given their roles in the Fire Emblem series, many fanfics explore how ike and marth become friends, support each other in battles and in their respective kingdoms' affairs.
I haven't read a specific 'South Park Ike x Firkle fanfic' yet. But generally, in South Park fanfics, the relationship between characters can be explored in really interesting ways. It might show a different side of Ike and Firkle's personalities. Maybe it delves into their friendship or some unique adventures they could have together in the crazy world of South Park.
One common theme could be friendship development. Since they are two characters, their relationship might start as acquaintances and grow into a deeper bond in the fanfic.
A great place to start looking for 'ike x marth yaoi fanfic' is on Tumblr. There are many fanfic writers who post their works there. You might come across some really interesting takes on the Ike - Marth relationship. Some fanfics might explore their relationship in a more angsty way, dealing with the difficulties of their positions in their respective worlds while still maintaining their connection to each other.
One common theme is the exploration of their unspoken feelings for each other. It could be about how they gradually realize their love in a world full of challenges.
Common themes in 'ike x marth yaoi fanfic' include loyalty and sacrifice. They are both characters with strong values, and in these fanfics, they might be willing to sacrifice for each other. Also, the theme of self - discovery is present. As they explore their relationship, they also learn more about themselves. For example, they might discover new aspects of their personalities that only come out in the context of their relationship with each other. This can be really interesting to read about as it adds depth to the characters.
Another theme is growth. Both Lucina and female Robin usually grow as characters. They might learn from each other's strengths and weaknesses. Like Lucina could learn more about strategy from Robin, and Robin could learn about courage from Lucina. It's all about how they become better individuals while being in a relationship with each other.
Well - developed characters are key. The author should really understand the personalities of both Female Robin and Lucina from the game. A good plot that fits within the Fire Emblem universe or gives a new spin on it is also important. For example, a plot about them teaming up for a difficult battle and how their relationship evolves during that time can be really engaging.