We definitely need more haircuts, and the option to dye your hair different colors rather than simple blonde, black, brown. I have brown hair irl but got white highlights recently (upon girlfriend request), so I think adding highlights for both genders would be cool too. And because Hoenn is based off of southern japan, I see a lot more funky styles with headbands, jumpsuits, and other stuff like that. My prediction for female clothes;
*Hats will be bandannas, no hats, and beanies.
*Tops will be sleeveless sweaters (famous in southern japan culture), the default shirt/skirt, and long sleeved shirts.
*Bottom wear could have skirts, shorts, and jeans.
*Shoes will have boots (famous in southern japan culture), tennis shoes, and sandals (for all those dang water routes).
*If they include competition clothes where you can make your own fancy outfit I imagine dresses with lots of customizable features.
My prediction for male clothes is;
*Hats will have headbands, no hats, and casual baseball caps like before.
*Tops will have shirts with bubble vests (again another style in southern japan), default short sleeved trainer shirt, and jackets.
*Bottom wear could have shorts and jeans.
*Shoes will have boots and tennis shoes.
*If they include competition clothes you can make your own fancy outfit I imagine suits and ties (oh yeah ;D).
These are my predictions, this is based off of research of southern japan clothing, but to be frank I don't know if they will even add character customization in this game because it is a remake and they want to make it like the previous game as much as possible. I hope they add more customization possibilities though, I liked making my trainer look like me and it made me feel like I was in the game. Anyway I hope they add more customization rather than take it away. Feel free to add on to the list for your prediction of clothing.
*EDIT: I saw my thread was added to another's so I restated some of my opinions in this post.*