Idea 1.

This one is pretty simple. Yoriichi Tsugikuni is reborn in the AGK world and aids Night Raid and the Revolutionary Army against the Empire. He'd be a great counter/antithesis to Esdeath. Both are natural born prodigies with contrasting beliefs. He could even train Akame and Tatsumi to be better swordsman. Especially Akame when it comes to her Teigu. Seeing all the corruption would harden his heart, which would make him far more deadly as a warrior and adversary.

Idea 2.

The Hashiras are reborn into the AGK world and aid Night Raid and the Revolutionary Army against the Empire. The reason for this is because I'd prefer characters that aren't the main protagonists as protagonists. I can definitely imagine them training Akame and Tatsumi. All the Pillars are swordsman after all. They could possibly even help Akame with her Teigu and Trump Card since its abilities are similar to a Demon Slayer Mark. Pairing wise, Mitsuri x Obanai is a must. I can see Mitsuri teasing Mine when it comes to her feelings for Tatsumi. Also, I can definitely see Shinobu and Chelsea working together when it comes to poison. Speaking of Shinobu, should someone put Merraid Oarburgh in a story, there needs to be interaction between the 2 characters for 2 primary reasons: both are butterfly based and specialized in poison. In addition, like Mitsuri, I can see Shinobu teasing Mine. Sanemi and Akame could have some chemistry (not romantically) when it comes to their younger siblings. Leone and Esdeath would probably have a crush on Rengoku for his fiery personality.

To sum it up, it would mainly be Rengoku and the Hashiras that died in the Infinity Castle Arc to be the ones reborn.

Idea 3.

If Muzan and the other demons make an appearance, I think it would be a neat idea if they worked to take over the Empire. This could cause a war that will either strengthen the empire or severely weaken it. Reason for this is because I can definitely see Muzan being disgusted by Honest, Syura, and the Emperor, (the guys who control the Empire) so I can't really see him working together with them. I can, however, see him working with the Jaegars. I can also see him being interested in Danger Beasts and Teigu as a bid for him to gain immortality.

Idea 4.

I think it would be cool if Hinowa Ga Crush was added into the mix as well. This idea was inspired by the fanfiction Fate:Kill by Parcasious. Unfortunately, I can't think of much for this one, but I think a fight between the Pillars and Ten Stars of Tenrou would be pretty interesting.

*Rengoku vs Yomihime (Battle of Flame)

*Gyomei vs Amateru (Battle of Warrior Priests)

*Obanai/Tengen vs Kururiya (Battle of Ninjas)

*Sanami/Muichiro vs Moegi (Battle of Wind)

Idea 5.

This one is partly inspired by Idea 2. This one will be mainly revolved around Rengoku, because he is my favorite character in Demon Slayer. Rengoku's whole philosophy is protecting the weak and living life to the fullest. The world of Akame Ga Kill is filled with people that abuse and crush the weak, with the greatest among them being Esdeath. This would go against everything Rengoku stands for, so I can see him allying with Night Raid and Revolutionaries. If paired with Night Raid, I can definitely see him getting along with Tatsumi, Leone, Sheele and Bulat foremost of all of them. In terms of ability, I believe that he still has room or improvement. After all, Akaza stated that he was close to reaching his full potential. Not to mention the fact that his fellow Pillars (most of them anyway) were training to awaken their Demon Slayer Marks. Speaking of which, I believe it would be pretty cool if Rengoku worked to create his own Demon Slayer Mark. I know we never saw him with a mark, but he definitely has the potential to get one. If I were to give it an appearance, it would be a flame shaped mark, which is pretty simple to picture considering his Breath style and theme. He could even develop his own techniques.

In many fics I've read, Esdeath is mostly paired with the character from a different franchise, so I think Rengoku would pretty good fit. Both could have some pretty interesting chemistry considering their background, themes, and philosophy. Rengoku was raised by his mother to believe that the strong should protect the weak while, Esdeath was raised by her father to believe that the weak die and the strong survive. Rengoku has the theme of fire, while Esdeath has the theme of ice. Fire and Ice has always been a interesting chemistry throughout storytelling, kinda like that of the tiger and the dragon.