The new mustang would look beautifull...
Lowered a bit, with some kind of functional hood scoop, white with blue shelby stripes, retro wheels, hood pins would be cool, a nice chin spoiler like the old BOSS mustangs, and a cool ass engine.
Maybe a new bigger V8 like prowl suggested. Or just a 4.6 stroked out to 5.0 liters or so with the supercharger from the cobra on top of that. They could always just shove the new GT engine in it. :D
It should never ever in any way shape or form be anything other than rear wheel drive however. Anything else would be an insult and disgrace to the heritage of the car. (ok maybe it would be bad ass, but I just don't think you could justify it, not on a stang, it's just wrong.)