I doubt we're seeing a redesign this year. Although I agree it's probably about time. I'd think next year would be it. Depends on what their next objective is with a redesign (TB3, new 'lightning with audio' coming to iPhone next year, removal of HDMI, USB-C, etc.). Keep in mind, it's not just about a "fresh new look", it's also about ports and stuff. Apple is very systematic, they just don't say "oh hey - it's about time to change the design, let's shuffle a couple ports around and change the look".
Can anyone shed some light on what Skylake allows manufactures' to do when compared to Broadwell? Thinner, smaller, anything that would allow for more room in chassis, etc?