Interactive Animations(or VFX):
Honestly, this would be kind of cool, the character's (player, NPC, etc) clothing and hair(if they even have any 🤣) corresponds to your breathing style. For example, people who wield water breathing should have their hair and clothes flow smoothly, just like water. When you side dash the clothes should be like this too :))
I feel like when players are fighting and they are using a cool move everything around them in their screen just blurs out(or something to make your move stand out). One element I notice animators often use is that they would have the demon slayers' moves flow across the screen like a 2d animation, but with 3D graphics, to make the audience feel like they are being hit.
One thing that makes characters look flashy is that their avatar is sort of disappearing. For instance, the final form for rengoku, shown on youtube in the hinokami yt channel, can have the character's clothing brighten, burn, starts flickering really fast or turn into (the whole character) light(maybe flame cuz it is flame breathing) particles.
When you see demon slayer characters down slayer land onto the ground heavily you'll always see dark colored dust poofing out almost immediantly. This VFX might bring the enviroment to life.
Looking at the white colored trails in the 9th form, flame breathing, it should be more darker so that we can put more focus on the flames.
One thing I noticed in the tengen vs gyutaro fight is that their moves relfect of the ground. This will make it look like the moves are actually real and it is not fake.
When you ragdoll maybe you can add some animations from face plant to trying to get up.(I don't think this is a good suggestion tho...)
I don't feel like it is a good idea to have wings that are moving up and down attatched to the boss, since the boss is walking or running on the ground