If I have to pick only one, I’m going to go with modern crumple zones

But man, I do wish we had some kind of magical smart metal that could be as rigid as an old car for low speed collisions, but still crumple for more serious impacts.

Because when you drive an old shitbox like I do, pretty much any damage is enough to total it, and having to get a new car really sucks when the accident was minor enough that no one was going to get hurt anyway.