I'm really curious how the body-designer will work. I think the success of the game greatly depends on this part.
"Probably the biggest aspect of creative control you'll have is over headlights, tail lights, door handles, grilles etc (not only style, but where they are placed on the car)"
Have you guys played the game Spore? In Spore you can design creatures and their vehicles, buildings, etc. The vehicle-creator is easy to use and fun: maybe a similar system in Automation may work fine.
Let's see how the designer in Spore works:
1. The parts are divided into groups: base body shapes, which you can resize and combine free. How could it work in Automation? What about to split in two every main body shell, for an upper and a lower part - divided by the waistline. You choose a lower part design, which would be the base of the modell family: and you could put different "top end" to that. This way you could make the sedan, station wagon, coupe, cabriolet etc. version of the same car. Of course, you could modifiy the basis for each version: for the coupe you could make it shorter, for the minivan you could make it taller, stuff like that.
2. The basic vehicle shapes are "magnetic": you could attach the smaller parts onto it. It's a great way to place the handles, bumpers, lights, etc.
Spore vehicle creator gameplay:
http://www.youtube.com/watch?v=n6zcC0xc6IkI don't suggest you to copy the whole Spore vehicle designer into Automation, but I think it's a clever system and some points of it would be worth to try.
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Anyway, I think there is two way to make the body-designer interesting:
1. Make the building system complex. Give the player great freedom to shape a few basic model. I think it would be hard to make it bug-free and easy-to-use. Let's see the other solution!
2. Make a simple building system, with a few basic transform options (Spore-like), and add masses of models and car shapes to it. It 'would be a huge amount of work, but maybe you could find talented 3D artists how would be interested in your project, here:
http://www.smcars.net/