We have the technology to 3D scan a person to get a current set of measurements, and generate a virtual sewing mannequin. Make that mannequin as life like as possible, standardized, regularly updated, and available to all clothing manufacturers.
Let each manufacturer who wants to use it design skins of their designs, with as much of the physical properties of each garment imbued inn that mesh as the state of the art allows, for any garment, in any sizes, with whatever sizing nomenclature that they want to put on the market.
Let the customer browse, and decide on where to shop based on what they see. It would be nice to also walk into any shop with some clear ideas of what sizes you will be pulling off the rack.