I think that although clans are generally not a good thing to add (i.e. the transactions to get re-rolls, unbalanced buffs, etc), I think that adding a clan system that integrates with story mode & progression would be cool.
How this would work is through the beginning when you customize your character and you get to choose your name. You get to roll a clan (which doesn't just have to be clans from the demon slayer series, but even just random clan names would be cool, making it seem a lot more randomised), and depending on what clan name you get, you can get a different storyline with different quests available.
Like for example if someone rolled "xx" clan and started off the story by having their family disappear due to demons and thus starting their quest to becoming a slayer or if they rolled "cc" clan they started the story with being in a family full of ninjas.
Overall, I think if clans are integrated like that through different stories, it would not only make the game more fun but also a lot more unique.
TLDR; rather than clans giving buffs, they should be able to give different storylines & quests.