I'd love to see the option to have a 'Responsive Canvas', similar to the way the canvas works in Adobe Edge Reflow (which is now being ported to the new version of Muse). Obviously the responsive canvas isn't much good unless you can define breakpoints and layout/styling changes at those breakpoints, and having the ability to 'pin' items to the edges etc., so it's quite a big change. However, more and more tools are becoming available for true responsive design, and I'd like to know what plans there are for Sketch to stay in the game in this regard?