One of the artists for the game, Mark Van Haitsma, released character concept artwork on his website, showcasing some of the differences from the character artwork to their actual models. Sonny's spoiler folds were originally pointing down, but were later changed to point up. Additionally, Cortland's racing number was to be 4, but was changed to 191 for unknown reasons.
More content can be found in the localization files:
There was going to be a sixth Piston Cup race at a track named Pinehills International Speedway.
There could have been a race with Lizzie.
There was a mini-game where one can pave the road with Bessie. However, it was released in Java version of the game.
There were going do more Doc's Lesson events more to do such as a Tilting lesson.
There was going to be one more Radiator Springs Grand Prix race called "Radiator Springs Grand Prix 4: East Valley". What this "East Valley" area is is unknown, but it could've been a fourth area to the map.