Ok, I know there have been lots of different ideas about design, but bare with me.
I have a few ideas, and I’ll organize too 
- Car Design vs appeal
A lot of people were concerned on how the car’s design would be determined good for certain types of customers at certain eras in the game.
Well how about this - certain body style and fixtures will be unlocked as time progresses plus each body style and fixture type will have a different percentage of atractiveness to different people.
Say during 50’s if you pick a modern body, with modern fixtures, it will have 0 appeal to any group, so it won’t sell at all. But if you pick a 50’s style with 50’s style fixtures, your car will have a lot of appeal to certain groups. Then if you stick with 50’s styling, in the 70’s their design will be outdated and will have 0-20% appeal.
Plus, besides design, all groups would have requirements they need.
Maybe something like that:
Petrolheads will care about 0-60, handling and oversteer.
Eco people will care about economy, small engine and compact size.
Soccer moms will need bigger size, economy, reliability.
Students will want low cost, small size, reliability.
Speed freaks will want supercars and millionaires will want big and expensive cars. - Competitors.
There should be plenty. And different types too.
There should be corporations like VW, Toyota, GM, etc. But there should be small companies that specialize in one part of the market like Noble, Smart, Daihatsu etc.
Plus you should be able to see where do you stack up against each of them overall and in each niche of the market both globally and locally.
I had more ideas, but I’ve been trying to word this for so long, I forgot it XD
