Once every breathing is in the game you could add breathing variations, which I can think of two canon ones.
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Makomo's water breathing, which has moves focused on mobility and the effect idea could be based around bubbles (Taking inspiration from the final selection demon saying that Makomo was very agile, and the fact that in Hinokami Chronicles she has a ult with bubbles effects).
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Different thunder skill set, based by the fact that Zenitsu only uses the first posture and [REDACTED] posture, a variation could be added with the other postures instead depending of what we get to see in the movie from the 6th uppermoon.
Okay, unrelated but can we get Genya's fighting style? He is a very important character yet he receives nothing in these kinds of games, like a skillset based around his gun and a stat boost granted upon execution idk. (With this, I mean an actual skill set, not whatever Demonfall does)
A mistake that a lot of KNY games do is that they don't balance the Demon Slayer and Demon content. (They focus too much on releasing every breathing because of it being the popular faction that it gets to a point where everything that you can add are demon arts and it kills the game)
My idea is not a fix, the fix should in fact be to just fix the pacing so that there is not a dominating faction.