Well, if one looks at things logically, we can make a list of what's needed.
1: Sedan, convertible, estate, hatchback, and coupe versions of cars we already have. Yes, I know I've covered a lot, but there's a lot of cars that are only a sedan or only a hatchback. It's currently all too easy to run right into a situation of wanting to build something and not having a car to do it with.
2: Modern pickup truck. Could be done with one basic body style and lots of morphs, along with having 3 major versions of it: Single cab, cab-and-a-half, and double-cab.
3: More SUV's. I know they're mostly an American thing, but we have a choice between the 70's era SUV and the 2010's era SUV. We need some middle ground in the 90's for these things.
4: More wheelbase variety. We have the four categories, but it's a great way to show the current problem more than a sorting system. Wheelbase under 2 meters? One car. Wheelbase between 2 and 2.5? A good amount of cars. Wheelbase between 2.5 and 3? Just about the same as 2-2.5. Wheelbase over 3? Basically only the really old cars. Now yes, I understand, 90% of people will build out of the middle two categories. However, it's the same as the SUV or the Truck problem right now: there's simply not enough variety.
5: This is one that's a bit more of an opinion, but more morph variety in the vehicles we have. With a handful of exceptions, it really does feel like the only thing you can do to change some of these cars is to stretch the trunk a little and change the hood line.
6: More people-movers, both in the form of vans and minivans, as well as pointing back to SUV's for a moment. Most minivans and a lot of vans (and some SUV's) have seating for 6 or more people. We can't honestly have a car tycoon game where we don't have people movers. Sure, some effort was made to implement a few, and I'm glad to see it, but right now our choices are the 60's van (which admittedly is pretty cool), the 80's/90's van, and the 2000's minivan. Which, I suppose, is better than some of the other car categories.
7: This one's at the bottom of my list only from the point that we're not quite there yet. Cars of the Future. Currently, our last vehicle unlock is the SUV at 2008. We have no cars for 2010-2015, and we certainly don't have cars up to the game ending of 2020. Modern cars don't look quite as wind-tunnel-rounded as cars from 2000, they've come back to an edgy, more aggressive look. As for cars of 2020, just about anything could go.
8: Filling out the eras. Right now, there's a few big gaps in the timeline, though less than before. 1940, we have no coupes, just two sedans. 1950's fairly well rounded. Small car (lots of choices), big car with lots of choices, and a period-accurate truck. 1960 shows promise, but may need some work. 1970 needs some work as 3 of the car choices are the same car over three different wheelbases (and one is missing the 4 door sedan version). 1980 is well rounded (sedans, hatches, coupes, truck, estates, wheelbase variety). 1990 needs more trims for the cars it has (too many are just what you see on the tin, instead of offering 2 and four door variants). 2000 needs some variety. 2010 really suffers as there are too many wheelbase doubles with no variety (2 mid-engine coupes, two sedan/wagon) compared to the rest of what you get (Coupe/convertible sports car, SUV, Sedan/Hatch) and could really benefit from some trim variety (SUV gets a pickup body, sedans get 2 door versions). 2020 has no cars from the previous 10 years, indicating a major hole in our designing ability.
I know, it's a big list and a lot of rambling, but it's just stuff I've noticed. Most of this could be fixed with new versions of what we already have, a few basic additions to what we have to fill some gaps in, and some tweaks to the morphing on the cars to offer more variety. I have no problem with only having one sedan for a specific time period if I can make that same sedan look like four or five completely different cars.
Add to the list some specialty versions of cars (long-wheel-base sedans, bigger trucks) and Kei cars and the design list would be filled out rather well.
That, and I'd love to have a two-door version of that Cadillac-styled big four-door sedan, the one they added the drop-top to in this round of updates.