So for Hantengu I think it would be really cool if you added in the clone ability. Like maybe the more clones he/the player spawn the smaller and faster they get, but their health gets lowered. Maybe they can control the clones, like switch between piloting one and can use the one they're piloting bda. Oh and when the player/Hantengu's health gets to a certain point the clones could disappear and spawn Zohakuten/the player with the drums and use the root dragon bda, then the original body could turn into that one clone when tanjiro was about to cut off the main bodies head.
For Gyokko, you could give him/the player the pit teleport ability, like when it's activated wherever they are looking a pot will spawn so they can dodge an attack. You could also have it where the player spawn a pot (depending on which attack) from their arm to send out water (where they trap someone in a bubble and they take damage since they're drowning), tentacles, the fish that shoot spikes, that giant fish creature thing, ect. Each vase corresponds to a certain ability. But to finish off Gyokko you have to destroy his main vase before the molting ability happens, where both Gyokko and the player get a massive buff for a period of time mixed with blue scales on their appearance.
since they were both in the swordsmith village maybe they could be part of an update that includes their boss fights, bda, love breathing, and mist breathing, or even just cool new features, like maybe you could get some of Gyokko's scales that can be used to upgrade your nichirin.
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