One idea could be to explore how Tatsumi and Mine handle the news of her pregnancy during the chaos of their fight against the corrupt empire. Maybe they initially worry about how their child will grow up in such a dangerous world, but then find hope in the idea of creating a better future for their family.
Growth and redemption are also themes. Tatsumi and Mine have both done things they might regret, and fanfictions can show them growing as characters and finding redemption. For example, they might try to make up for past mistakes by helping those in need. Additionally, friendship is a theme, as their relationship is also based on a strong friendship that has developed over time.
Mine, on the other hand, would probably become more reliant on Tatsumi. With the physical and emotional changes of pregnancy, she would need his support more than ever. This could deepen their emotional bond as they navigate through this new chapter of their lives together.
One possible plot could be Tatsumi and Mine going on a secret mission together. They might face a powerful and unexpected enemy, and during the battle, they discover new abilities within themselves and grow closer as a result. Another plot could be a story set in a peaceful time after the main events of Akame ga Kill. Tatsumi and Mine are trying to build a new life, but they still have to deal with the traumas of their past.
One way their worlds could collide is through a crossover event. Maybe a powerful villain uses some sort of dark magic or technology to bring characters from both Naruto and Akame ga Kill together. For example, they could be forced to fight in a gladiator - like arena where the only way to survive is to team up with unlikely allies. Naruto's skills in ninjutsu and his strong will would be an interesting contrast to Mine's use of her Teigu and her sharpshooting abilities.
Akame ga Kill novel's plot is centered around the struggle between Night Raid and the corrupt powers. Night Raid is a group of assassins. Tatsumi's entry into this world of conflict is a significant part of the story. As the story progresses, the battles become more and more fierce. The relationship between different characters, such as the complex feelings between Tatsumi and Esdeath, also adds depth to the overall plot. There are also political intrigues and power struggles within the government that Night Raid is trying to overthrow, which makes the plot very engaging.