I assume the t-shirt will be the std body garment and what we think of as tracksuit trousers will be the std bottoms, most probably made from a fabric sourced from kelp or mushrooms or the like- I expect it to be very breathable and take a 3 minute superwash in the new machines we will have which will be more like a conveyor-belt diswahser than the washing machines we know now and to dry in 15 mins, or under 2 in an air curtain. People wearing "traditional" Adidas Samba and Air Jordans will be seen as having impeccable taste.
I believe there will be a major societal fragmentation with the very wealthy continuing to wear woollen and linen clothes in an interpretation of the tracksuit, which will be the new version of the business suit, but with small lapels and a very fitted look which most will be able to pull off due to very strict diet. Not sure about buttons. They might be out.