Alright, now with this suggestion it's quite hard to achieve. We all know that Akaza was not able to regenerate his head, and had to "give up" to die. And upper 6 required both being decapitated at the same time, I'll add both to this suggestion. What I mean is that with other boss fights in games there's never realistic deaths, you just hit them until their hp is low. With this you can't do that with certain demons.
Akaza
Wherever you spawn to fight the Akaza boss, you have your fight with Akaza and get him low, now there would be a cutscene with whatever breathing style you have and you decapitate him (will be a diff animation for whatever breathing style you have) or they could reenact when Tanjirou leaped upside down and did it but with whatever colours your breathing style is. We see Akazas body go "limp" but still stand, he then enters his iconic pose like he did in the manga. You then cut to you regaining your stance quickly and start the breathing animation. Then the cutscene ends and you get to play back again, and another fight ensues as he still has no head. After you get him weak, he falls down limp. On the top right or left where the quest text would be, it says "Chain 0/3" and a cutscene ensues where your character turns its head to look at the chains around the area. You have to keep hitting and knocking him down or else he will get back up and you have to hit him again. This ensues until you place the 3 chains over him, and then a cutscene appears with a shaking screen. The camera cuts to above where the sunrise is coming up, and light slowly envelops the battlefield where you and Akaza are. He let's off his scream that he used when being held by Rengoku, and he slowly disintegrates. While disintegrating you could make his head start to appear and half of it is Hakuji, this parts def a stretch. Imma write the second upper 6 part of this suggestion in a minute.





