Mustang Sedan, Take Two—I modified the rear bumper/apron assembly from my original chop below, with more sculpting matching up with the side contours. I think this lessens the bumper's thickness. I also made the roofline "faster" and added a dark-tinted roof panel, emphasizing the slope more. I think this makes it a bit more muscular. I also added a "5.0" badge to the front fenders.
Mustang Sport Sedan—Yes, the purists always hate these, and that's why Photoshop is so much fun. I started with an image of a Saleen-modified Mustang, hence the ribbed rear panel, quad exhausts, wheels and side skirting (although I modified the skirting, making it more "adult" and less boy-racer). I added the Mustang logo to the rear cove. I figured as long as I was committing blasphemy by adding the second set of doors, I might as well leave off the well-known Mustang medallion in this area (the gas cap on the original series) and use the prancing horse instead. I lengthened the wheelbase by about eight inches to make enough room for the rear door to be functional.
Would Ford ever build a Mustang sedan? I don't think so, but I don't see why they haven't utilized the rear-wheel drive platform for a sedan with a different name. It would make a nice Lincoln sport sedan.