Esdeath's character can be developed by exploring her backstory in relation to the thief. Maybe she had a similar experience in her past where she was an outcast like the thief. This can make her more empathetic towards the thief. The thief's character can be developed by showing their growth from being a simple thief to someone who has a greater purpose, perhaps influenced by Esdeath's grand plans. As they interact, both characters can change and adapt, making their relationship more complex and interesting.
One plot idea could be that the thief is initially stealing from Esdeath's domain. Esdeath catches the thief but is intrigued by their skills and instead of punishing them, forces the thief to work for her. As they spend time together, an unlikely bond forms between the cold - hearted Esdeath and the wily thief.
Tatsumi is very likely to be involved as he has a significant relationship with Esdeath in the original story. Her subordinates in the Jaegers might also be part of the fanfic. They could be shown as being protective or having mixed feelings about her pregnancy.
You can try looking on some fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics, and you might be able to find Esdeath and Tatsumi - related ones there. However, make sure to follow the site's rules and regulations.
One common theme is the idea of redemption. Often, the OC tries to bring out a softer side of Esdeath, as if redeeming her from her cruel ways. Another theme is the power dynamic. Since Esdeath is a powerful character, the relationship with the OC often explores how power affects their interactions.
I haven't actually read this 'love letter from thief x hiro fanfic' specifically. But generally, in a fanfic like this, it might explore the relationship between the thief character and Hiro in a very unique way. It could involve themes of unexpected love, adventure, and the conflict between the thief's nature and the feelings for Hiro.
First, you need to understand Esdeath's character. Know her motives, her desires, and her flaws. Then, come up with an interesting plot that either challenges her or plays to her strengths. For example, you could put her in a situation where she has to make a moral choice that goes against her usual nature.
The popularity might stem from the complex relationship between Esdeath and Tatsumi in the original 'Akame ga Kill!' anime. Their dynamic, with Esdeath being a powerful and somewhat cruel character and Tatsumi being the more idealistic hero, creates an interesting contrast that fanfic writers like to explore further in a romantic or more adult - themed context (while still keeping it within the boundaries of fanfiction).
Some common themes could be the exploration of Esdeath's character in a new light as a mother - to - be. It might also include how her pregnancy affects her relationships, like with Tatsumi. Another theme could be the challenges she faces during pregnancy in the context of the Akame ga Kill universe.
One popular 'Esdeath x OC fanfiction' is 'A Frozen Heart's Thaw'. It delves into the relationship between Esdeath and an original character in a unique way. The OC in this story has a backstory that intertwines with Esdeath's cold nature, gradually melting her heart through a series of adventures and emotional exchanges. Another great one could be 'Esdeath and the Unlikely Ally'. Here, the OC is a character with a different ideology than Esdeath at first, but as the story progresses, they form an unexpected bond.