One theme could be redemption. As mentioned before, Karai has been involved in the Foot Clan's wrongdoings. April could be the catalyst for her redemption, showing her that there is a better way. Another theme could be friendship against all odds. Their different backgrounds and loyalties make it seem impossible for them to be friends, but the fanfiction could explore how they overcome these barriers.
There are also fics that play with the idea of them having to keep their relationship a secret. Given their different backgrounds, secrecy might be necessary, and this can lead to a lot of tension and drama in the stories. Also, some fics might explore how they help each other grow as individuals while being in a relationship.
One possible scenario in a fanfiction about April and Karai could be their relationship evolving from rivalry to an unexpected alliance. They start off as enemies due to their different allegiances in the TMNT 2012 universe. April, being associated with the Turtles, and Karai, with the Foot Clan. However, as they face common threats or discover shared secrets, they might begin to work together. This could lead to some really interesting character development for both of them, as they learn from each other's strengths and weaknesses.
A great Karai x April fanfic often has well - developed characters. The writer should understand the personalities of Karai and April from the original source material and build on them in the fanfic. For example, if Karai is portrayed as a complex and conflicted character in the show, the fanfic should maintain that essence while exploring her relationship with April.
Check out Tumblr. There are often fanfic recs (recommendations) on Tumblr blogs dedicated to the particular fandom. Some fans are really good at finding and sharing great Karai x April fanfics. You can also follow relevant tags such as #KaraiAprilFanfic to stay updated on new stories or discussions related to this pairing.
Karai could start off as a cold and calculating character, but as she spends time with April, she might become more empathetic. For example, seeing April's kindness towards others could make Karai question her own actions.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including Karai x April. Just use the search function on the site to look for relevant stories.
Their different backgrounds make them interesting. April is a human with her own set of values and skills, while Karai has a complex backstory in the ninja world. Their clash of worlds can create great tension and drama in fanfiction.
One plot idea could be that April discovers Karai has a secret soft side. Maybe she stumbles upon Karai helping some stray animals in the sewers. This could lead to an unlikely friendship where they start to understand each other's motives better.
One common theme is friendship. Since they are both characters in the same universe, many fanfics explore how they could become friends despite their differences. Another theme is rivalry, as they could be seen as rivals in some aspects of the story.