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.
Idea 2.
Just like with Yoriichi Tsugikuni, 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 weren't the main protagonists as protagonists. I can definitely imagine them training Akame and Tatsumi. All the Pillars are swordsman after all. They could maybe 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 would probably have a crush on Rengoku for his fiery personality.
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.