If the redesign is significant as everyone is really saying it is, it shouldn't be that hard.
I disagree.
Lets look at it's 2014 specs. The current model has a curb weight of 3,618lbs. That is more and/or equal to a lot of the big American boats that came out of the '70s and '80s. Considering that previous generations had have as much as 1,000lbs less in weight, there is no reason why they should not find some place that's able to reduce the weight, and if it takes make the car very small, then that's what it takes. It would be more nimble, and faster and bring back what it meant to be a pony car.
3,600-800 = 2,800. You want a Ford Mustang with a 5.0L V8 to weigh as much as a Subaru BRZ or Honda S2000 while abiding by the same legal safety standards. In order to do this Ford would have to make choices in the design of the car that would alienate 90% of potential customers.
The cars from the 60's-80's didn't have anywhere near the same safety regulations. The car needs airbags, the car needs a strong structure with crumple zones, customers demand air conditioning as standard rather than have it be an option that adds weight, the customer demands a soft ride with lots of interior space.
In order to house a 5.0L V8, crumple zones, air bags, radios, air conditioning, comfortable seats, and sound deadening the chassis needs to be made big. A big chassis that isn't made of expensive exotic materials is a heavy chassis.
In order to satisfy the consumer demand for the Mustang, the car cannot try to be as small and light as a BRZ. If they tried to make the standard car 800 lbs lighter by stripping it out or making it smaller, less people would want to buy it.
The overwhelming majority of Mustang buyers want the weight and comfort. People who want a Mustang to weigh as much as a BRZ represent an extremely small portion of the market.
Why would they listen to the "I want the new Mustang to be a 'true' pony car!" crowd? The money is elsewhere.