I would like to see the paint process extended.
Right now you have just the undercoat (prime) and the top coat (base) and their drying stages. In the manufacturing I seen for car paint you have many more levels from phosphate cleaning stage, e coat, sealer, sand booth, before you even hit prime coat.
Most of today car paints are 3 layers. So at the very least I would like to see clear coat added as the third layer. You could have 2k clear coat (2 part clear) added as a quality upgrade to last paint booth.
Maybe even separate the oven into multi stage ovens.