Going off the concepts i have seen, the 2015 looks like a 2013 fusion, with a fast back, 2013 style (kind of) tailights, and a few bulges in some panels. I think the car will be slightly shorter than the 2014 though.
Will be a futuristic looking retro re-do, if that makes sense- Like if a 69 mach 1 had sex with the 2013 fusion = 2015 Mustang