I'd love them to sell By Terry, so I didn't have to wait for hours in SpaceNK every time while both assistants are having intense conversations to expensively-dressed women. How many times have SpaceNK lost a sure-fire sale while the rich women get some samples and leave without buying?
I also wish they'd sell Armani cosmetics. It's a brand with limited retail outlets (for exclusivity, presumably), but all the stuff is lovely.
Fashion for the under-60s would be nice, too. How about some Abercrombie & Fitch?
There are some wonderful costume jewellery companies, like Les Nereides, or Reminiscence, which only have a shop or two in London (both French).
Or toiletries/ perfume companies like Ortigia, Patricia de Nicolai, Santa Maria Novella?