Mmmm, actually it could be a lot of stuff.
Hear me out:
Idea 1: Every fashion design becomes an avatar(or personificiation), of that fashion design and the fashion maker can use them as construct for prestige, and other stuff, like power, fighting and money. Etc. The more popular, and astounding the design the better.
Idea 2: It could be that warriors and people like to look nice during battles and nowadays, so they develop in the fantasy setting the technology to add above their armors and protection, the clothes as a skin. People only see them wearing fashion clothes but, but in reality they are wearing high armor, and defende tools. You could go that this began as an strategy of some offshot army tot rcik theenmy and have evolved trough the years in some sort of culture stuff, and fhasion stuff.
Idea3: Another idea is that the clothes can be something more like clothes, so the fashion deisgners in that world have the power to turn their clothes into other stuff, so gloves can became knifes to cut food. Scarf can be used to cut branches, and hats can be used to termodinamically change the temperature of the user,so they dont get too cold or to hot. You could also make the clothes some sort of communication centes, like they can write in their wrist a mesage and this message will appear in the cloth of someone somewhere else.
Idea4: Same idea as above, but every fashion design cloth have the functionatedly to work as a pocket dimmension, so the people can store their stuff there or use them as a mini dungeon, of small dimmension, with a diferent ecosystem following the themew of the deisgn or overall the design itself.
Idea5: All fashion design clothes, are alive in the fantasy seetting, but they can exaclty express themselves, unless they are beign weared by someone worthy, or their creator see them as a living thing, or they are put in a doll, or mannequinn.
Idea6: All fashion clothes have the power to interact with an element, any element beyond the ocassional, fire, water, air, earth. Things like grass, butter, beer, etc, count as an element, this helps because this is the only way to interct and have peace talks to people of other regions that live in places with these themes. So people of the fantasy world, force people to end there, because they need someone tlaented to design this types of clothes.
So thre are my ideas for now. Hit me by dm or here later if you want help or something, i wouldnt exaclly like the burden to be a co-writer, but i can participate as consultant in the little knowledge i have of fiction, and ideas. My only condition is that because of my work schedule, is probably i will stop interacting at times. Or have to abandond the project. But if you are ok, i can help you with this. Also my english is my second language so expect a lot of typos. A LOT.
edit.
Lol lol ok hear this one out.
Idea7: How about the world is at disaster and the wars have caused a lot of problem to the fanasty setting so the nations across the globe have make and accord, to substitute wars and conflicts that affect cityes and countries with fashion designs and beaty pageants. So this become like the best diplomatic way to resolve settings. But because some people are bad at design, they kidnap people of earth *cof* i mena recruit people from eart, to make the designs for them and be their forced consultant in this area. Wil, be interesting because this being an isekai world, they could have like dram like materials, gattered for fanasty beast an animals. And the isekaid person, will have to use his knowledge, and the new materials to make astounding stuff. To resovle all posible conflicts, and all the drama.
