I don't disagree that these things are possible. But, they are not common and I don't believe that such modifications fit the devs vision for the road cars. I've seen a twin engine Hyundai before. Since that's *possible*, I suppose we should be able to do that in the game too, according to your logic? What is possible and what is plausible are two different things.
Like I said, if they were meant to be full-on race cars they would have rollcages and no stereos. Yes, you can take your grocery getter to an autocross and not worry about a rollcage. Yes, you can put on aftermarket suspension (which is why I suggested somewhat adjustable ride height, damping, etc). NO car can get the granularity of gear ratios and diff ratios that LFS offers right now. "Road cars", such as the ones in LFS, shouldn't have such things adjustable either. Who in their right mind would spend the money on a custom transmission or diff and springs that are EXACTLY the perfect ride height and stiffness for a particular track, but then not bother with a roll cage, leave the stereo intact, leave the engine bone stock, and keep the car on street tires?
I believe, as I think that most LFS players do, that the road cars are meant to be lightly modified daily driver cars. While some adjustment would be welcome, they should not be treated as race cars or cars with huge amounts of money in the suspension/transmission/diff/etc. This just isn't a likely scenario and is the whole reason that we have multiple classes for each car.
Simply put, LFS needs some cars that are less adjustable and easier to setup, to make it easier for people who are just starting out, to keep the fields even, and to stay true to what a road car is. Not to mention the fact that limiting setups could go a long way towards balancing the classes. I would certainly welcome a new group of cars though. Something between mostly stock road car and the all-out GTR cars that we have now. I would also support opening up all the cars to use slicks, as you mentioned. I think this would be good for everyone, even though you'd very seldom see the street tires used if that were to happen, IMO.
And we still need forced setups, or at least some way to have a "showroom stock" class race.