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.
I'm not sure specifically about 'lemon' fanfiction as it often involves inappropriate or adult - themed content. However, there are many regular Tatsumi and Esdeath fanfictions on various fanfiction platforms. You can try Archive of Our Own or Fanfiction.net and search for their relationship - based stories that are more in line with general storytelling and character exploration.
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).
One key element is the clash of their worlds. Naruto is from a world full of ninjas and chakra, while Esdeath is from a more medieval - like world in her original story. Their different power systems and how they interact with each other are often explored.
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.
One great Naruto and Esdeath non - lemon fanfiction is 'Cross - Worlds Alliance'. It focuses on their unexpected meeting in a unique setting and how they team up against a common enemy. Their different powers and personalities create an interesting dynamic in the story.
If Naruto is Esdeath's brother in fanfiction, they could embark on adventures across different dimensions. Since Naruto has experience with his own world and Esdeath with hers, they could explore new lands together. They might encounter strange creatures and powerful enemies. Along the way, they could learn from each other's fighting styles and abilities, Naruto teaching Esdeath about chakra and she teaching him about her own brand of combat techniques. And they could also be on a quest to reunite their fragmented family or uncover a long - hidden family secret that has the power to change their worlds.