I'm not going to give a definite answer. Reason is because you never know who will pull off the next best thing that people will want to wear. It can be from anyone from any place. I think a person who ends up making the next fashion trend or being the next trendsetter will have to have the following:
* come up with a great outfit
* wear it amazingly
* show that this look can be amazing for anyone of any figure
So with this said, I'm thinking of someone from the usual suspects: America, England, (especially) Paris, (especially) Italy, maybe even Japan as well. I am always fascinated about Japan, but I fear anyone from there could bring about the next best thing that people the world over will awe over. I'm not counting the Harajuku Lovers deal that Gwen Stefani brought about as far as Japanese style, but a Japanese designer trying to come up with something amazing than any fashion American or European. I'm sorry... I can only really think of the United Kingdom, France, and Italy as coming up with perhaps the next best looks for women and/or men. I'm thinking about this as far as most fashionable looks in the world, not just my native America.
So I'd have to go with styles and stylists from the three countries I've mentioned. That is, unless someone else comes from out of the blue from a different nation that comes up with amazing style. This, even though I've recently became a fan of the Domincan Republic's Oscar de la Renta.