I currently own a 2006 GT hardtop, which I love and I'm not in the market for a new one at this time. After seeing some examples of exterior body work on the already nice looking modern incarnation of the Mustang, I wish Ford was still a little less "pedestrian".
Changes:
GT350 front fascia and hood
Galpin Rocket rear quarter "dual slit" side panels/hips/ducktail.(without the stupid Rocket emblem off course)


Probably the biggest issue I have with the car is the rear deck being pushed forward and down. It's just screams for a ducktail to chisel in the rear lights and come out a little towards the bumper. The Rocket does this very well and the more pronounced side hip leading up to the tail doesn't hurt either. Of course, I can't stand the whole front end of the Rocket or the hood, but I do love those curvy hips and ducktail.


I think the combination would look nice and give the car more exotic and aggressive looks.

 

Beauty is in the eye of the beholder. I love my 2015's design, just as it is.

 

I don't think the GT350 front fascia is more or less "pedestrian", its just different. I am quite sure both the GT350 design and the stock one were in the running for the standard design. Either design is just an evolution of the 13/14 design.

Same with the hoods two smaller heat extractors on either a side of the hood verse one larger in the center, just different. I would have liked to have seen the area between the two spears be even with the top of the spears giving the hood a cowl induction scoop look.

The agressive look of the back of the S550 is what attracted me to the car in the first place, I never cared for the slab sided look of the S197. The rear deck lid already has a duck bill shape to it and I have seen several aftermarket one that accentuate the shape and look good. Can't quite figure out the blade spoiler they put on most, glad my Performance Package car doesn't have it.

 

I like it, but in my mind anything is better than the fusion front and the terrible droopy back end of the S550. I can't wait for the '17 refresh and '19 re-design so I can start liking the Mustang again.