I had the pleassure to work in some Hot Wheels projects.
As usual I started with some hand sketches provided by Mattel, and then created a 3D model for production and prototypes.
During the model creation process I always have to take some technical and aesthetics decisions to preserve the design intent and deliver a model suitable for the next step.
The software I used for this kind of project was: Rhinoceros 3D, T-Splines and Shape Modeling plugins. Modo and Solidworks were also used in some special cases.