What comes to mind is the Citroen 2CV, 500 and 600 air cooled twin. The Fiat 500, again 500 air cooled twn. Subaru 360, same just smaller, oh yes the DAF, a Swedish car with twin variator transmision.
There is a kit to make a VW air cooled into a compressor, by making two cylinders pump air. The same can be said for taking two cylinders and disconnecting them, making a 60 horse into a 30 horse. Cant go on the freeway, but city mileage would probably be good.
The old 70's Tri Magnum took the concept of hanging a motorcycle off the back of a kit frame. Fiberglass body and the sound of an F1 style motorycle howling behind you.
One of my favorite concepts was the Suzuki GSXR-4, take a 170 HP motorcycle engine and put it behind the driver, then use a Quad style rear axle. A mid engined two seater that sounded like an F1 car. You can tell I like a car with 5 or 6 speeds and an engine note, lets you know your driving, not sleeping behind the wheel.
Oh yes, in the 60's there were a bunch of Italian 500 and 750 cc, little touring coupes. The Honda 500 and 600 R, coupes.
There is a DMV exception for Cycle Cars. It seems this is the best way to make light economical car-cycles. Now with the SUV reenforcements, economy cars are nearing 3000 pounds and things like the New Mustang comes in at 3800 lbs for the V8, 4100 for the convertible.
The Smart car was supposed to have a diesel twin, but got a 1 litre 3. I rented one to use on a trip to San Francisco and loved it. It was fast, people look at you and smiled. We challeged each other to find the smallest space you can park the car in. I got 28 city and 38 highway. Not close to book, but its all hills in the Bay Area.
I would love to see a return to 500 and 750 two three and four cylinder cars, perhaps even a tiny turbo rotary.