Then they'll have to kill the brand.
The new Mustang is the ultimate Mustang, the quintessential execution of the Mustang essence. It is also likely the ultimate Mustang, as in the last in the series.
Where does Ford and it's retro-addicted styling honcho J Mays go from here? The Downshift Retro Roadblack law says that once you've committed to homage, you can't go forward again. With the 2005 Mustang, Ford is saying that the best Mustang design -- indeed the soul of the marque -- can be found in the mid-60's models. There's not a single meaningful new design element on the entire car. It's going to be awfully tough to break out an all-new Mustang to follow.
Say what you will about the new Corvette (and we will in due time), Dave McClellan and his crew have never played the nostalgia card as a one-shot boost to sales. Each generation lives or dies on its own merits, while still maintaining the family bloodlines.
Maybe Ford has a retro-cyclical plan in mind. This new version is a 60's do-over. The next generation will bring back the kamm-back, big-butt 70's look. That'll follow with the "new" Mustang II. Once around with the 90's busy look and we can return to the 60's.