First the don'ts ...
Don't change the size for obvious reasons.
Durability and the ease of maintenance (e.g. don't have to pull engine to change spark pugs) are two things that make these trucks so valuable. Don't change either.
The changes ...
Put in heated mirrors and seats. Crying out loud. Every car I've owned during the last 10 years had heated mirrors. This is a truck that was purchased to go through weather. Cost has to be nickels and dimes.
Fire everyone who was involved in the rust problem. Revisit and restore the broken quality control programs.
Look into better streamlining and wind management. Stremlining will help mileage. Stop wind beating the driver in the face. Stop all the whistles and other noises that we all complain about.
Auto window on passenger side, and, auto up and down.
Get rid of center slide window and put in power side windows in the access cab.