iron wren
I know they already plan to add subclasses like uzui's ninja slayer class, i think other cool sub classes could be dancing slayer like how tanjiros dad used to dance/move rhythmically to channel his breathing techniques, also maybe kanrojis flexible and agility based fighting style, shinobous poison, and gyomei just being a tank with larger weapons.
I think these are cool lore accurate sub classes that could be considered, sub classes dont only have their own unique skills that are locked behind that particular sub class but also have certain buffs or maybe debuffs to some stats. Like uzuis ninja could have increased speed/agility but less damage output or less tanky.
Same applies to demons, but i dont have any particular ideas for them.
ALSO please make the open world feel lively snd eventful, add in world bosses and raids that are hourly or so that have certain drops and make parts of the progression team based like final selection