I have problems grasping this "Story Now". I mean, I like giving my players agency and whatnot, but I also like to world-build. And the more I read about it, it seems that "Story Now" and world-building are opposite concepts.
I don't actually think that Story Now is necessarily opposed to some elements that are often considered 'world building' to at least a degree. Thus, for example, even fairly staunch advocates of Story Now 'Zero Myth' play would still say that you need a solid idea of the genre, and its good to understand the tone and general sort of content that will go into a game. At a slightly less far out position you could also simply take up a type of game where the fundamental questions are mostly determined by the genre conventions and similar things.
The oft-cited Cthulhu Mythos example works well here. We all know what the primary geography is in the CM world, its an early-20th-Century Earth with certain locations which form focal points, Eastern Massachusetts and NE, certain regions in the South Pacific, remote Antarctica, the Arabian Desert, etc. What exists in these places is fairly established, as a general thing. The creatures which are likely to be encountered, Great Old Ones, Elder Gods, the Great Race of Yith, Fungi from Yuggoth, Deep Ones, shoggoths, etc. are all pretty well known quantities in terms of how they function in the milieu. So a lot of 'world' is already 'built' here. This would also be the case in a Middle Earth, a Marvel Universe, etc.
Even if the world is less fixed by the conventions of a specific genre niche, Story Now is certainly not going to be hurt by using some pre-existing overall setting. You can do it in WoG as [MENTION=42582]pemerton[/MENTION] has, or FR, or 4e's PoL world. The key element is that there's no fixed story. It is only 'world now' to the extent that overly specifying the world beforehand can create constraints which are then hard to break when the story would be better for it. Different people feel that there are different ideal balances. Dungeon World wants 'a map with holes in it', zero myth advocates for no map at all.
What is unlikely to be considered Story Now is a game where you have a whole bunch of encounter areas that are already set up with pre-ordained elements which focus on things that their author wanted to have in them. The things that show up in the Story Now story are things that the players have indicated are supposed to show up, could show up, or would take the story in the direction they want if they did show up.