Ok here is what I think:
1) keep the s197 chassis and bring the "track pack" down to all versions of the mustang
2) Engines: 3.7L 280HP V6, 3.5L EB 365HP (Call this an ST), 5.0L 400+HP V8 (GT and tweek it with a shaker for a Mach 1 return) 6.2L NA 550+ HP V8 (cobra, be done with F/I for a change. I'm tired of the first thing out of the GM boys mouth being "Ford can't make power with out F/I")
3) Styling, change the car to a true fastback. go off the Iococca Anniversary Mustang. Just work on the lines to make it flow better
4) KEEP THE SRA SETUP!!!! A mustang isn't a mustang with IRS. This is what keeps the price of our cars down. Just add the track pack features and be done.
5) For pete's sake move the fog lights out of the grill and move them to the bumper. I have always disliked the fogs in the gill on the S197 cars. Look at how clean the V6 2010 pony package looks compared to a GT.
That is all I can think of. I LOVE my V6 S197 and hope to keep it for a very long time. Keep of the hard work Ford. We can't sit back and just hang out for 7 years like we have. The camaro coming back is the best thing to happen to the mustang in a long time. Now we need to get ahead of it and stay out in front.