Well of course the Mustang is going to get smaller, the camaro is so hence the Mustang will and also Ford is a key investor in a new metal technology that is lighter then aluminum yet stronger then steel. Its some sort of nanometal or something like that. If I can find the article somewhere I will post it up.
The next Mustang is going to be one hell of a car. If Ford engineers can build the IRS just as well as they built the live rear axle in these cars along with the 5.0 and the EcoBoost 6 and the 3.7L, its going to be one hell of a car, well balanced in all facets, straight line, corner carving abilities and interior quality. I think that where most of the global design influence is going to come in would be the interior since IMO only American designers should be playing around with the exterior to keep it truly an American design since it is an American Icon. Also according to J. Mays its going to be a departure from the current retro theme that the S197s have.