Hello, I’m a relatively new comer to this game.
One thing I would like to see added is the ability to modernize older body types to remove the safety penalty. I say this because there are currently 
only one small city car body (the mini) and I’m trying to build a prototype economy city car. Just a small button/switch/choice next to the body selection.
After reading the guidelines, here are my ideas to make it worth it:
Modernizing a design adds a significant cost/ production penalty to represent the extra effort it takes to make old designs safe.
The update shrinks the engine bay to show the added crumple zones.
I think this would add a lot to gameplay by allowing the construction of Retro designs that can still meet modern safety requirements.
It would also allow me to personally build my dream car (A fully upgraded dodge charger) 
While I somewhat agree that the penalty could be tweaked in some way, some bodies still have inherent safety issues due to their shape.
Most of the boxy bodies probably would have issues passing pedestrian safety now-a-days for example.
Maybe as the years go by, the old body types disappear and x years later re-appear under a “retro” tab, where the bodies are ‘new’ and don’t have as much of a penalty?
You need to ask yourself: why don’t we see any car maker today just take the old design and re-release it as a new model? There are so many people that would love to have a modern version with exactly the looks of the old classy one. Instead we get modernizations like the Mini… not even comparable! So no, I think the penalty probably is a bit too tame already, otherwise we’d be seeing this happen in the real world 
I think this won’t be as much of an issue once we have more bodies in the game.
The 2015 Dodge Charger is a completely different car from the 1970 model; only the styling is reminiscent. If there was a 10’s muscle car body in the game, you wouldn’t need to use a 70’s muscle car body to produce a modern remake.
What about morgan?I think that there should be a system that allows us to lower safety penalties(not completely remove them) by raising engineering costs and possibly some other penalties like higher production costs and smaller engine bay
I think it’s still possible though. We don’t see it today because manufacturers are trying to appeal to the current market. The number of people who want retro cars is sadly to small to cater to.
I agree that some older shapes would be inherently less safe, but we should be able to take steps to mitigate that. You can greatly raise the safety of an old car just by putting a proper seat belt and airbag set in it. Most of the danger came from the fact that most older cars had “optional” safety equipment, especially when seat belts weren’t mandatory. Having owned a 69 Mercedes, I know just how minimal the safety equipment is in older cars. In game, just replacing the standard 70s safety equipment with standard 00s equipment should greatly raise the safety of the car. The production penalty could represent redesigns for pedestrian safety as well, as parts of the hood would be designed to give in.
This does bring up another question of how this will be handled in the campaign mode. Will a very safe 60s car just slowly become less and less safe as the years go by?
The Morgan uses the same tech that in the 30s years, but modernized (for example, the same wood frame).
And the original Mini Cooper: the same body since 50s up to 1999-2000 (before BMW version).
Safety should be independent of time. It would allow cars to be comparable in terms of absolute safety between years. The safety penalty is probably just a warning that the body is, by design, unsafe compared to other available and newer bodies. If a car from the 60s provides at certain amount of safety in a certain crash, that quality of safety will not be different just because it’s 55 years later.
Which is exactly how it is handled in the game currently. A body will continue to improve in safety along the normal time progression up to (I think) 10 years after it becomes available and then stops progressing, i.e. it does not become worse. This doesn’t mean you can’t fit it with more modern trim options to increase safety further.