Until the recent rise of SUVs, this was the standard car design

The 4 door sedan with what designers call the 3 box style: 1 box for the engine, another one for the passengers, and a third for the trunk.
But there's a problem: you have a lot of wasted space on top of the trunklid that isn't utilized. There is of course a style that fixes that
The station wagon. This Chevrolet Caprice wagon takes up as much space on the road as a Caprice sedan, but it has much more cargo space and a third row of seats.
I can come with a pretty simple POD to ensure that the wagon becomes the standard design.
Most cars until the war had a vertical rear end, like this 1939 Chevrolet sedan. It's not a wagon, as it has a normal trunk, but it could easily evolve into a wagon. However, that same year, Cadillac ruined everything.
They introduced a new style of sedan which had this big trunk sticking out, before, this had only been used on coupes.
After the war, sedan trunks slowly started puckering out, like on this 1947 Chevrolet. And in 1949, the inevitable happened
It infected Chevrolet...
Plymouth...
And Ford
Ironically, that same year, Plymouth opened up the station wagon to the masses.
Before then, station wagons were made of wood and were extremely expensive, but in 1949, Plymouth built the first all steel station wagon, which was affordable to the growing mass of baby boomer families. But even so, people had taken to the sedan and wagons never could exceed 1/5th of the market.
Let us imagine that Cadillac never brought out that new look for 1939 and stuck with the vertical rear end and the rest of the auto industry followed along. Then in 1949, Plymouth could bring out an all station wagon lineup and set the trend for the rest of the industry.
You're probably wondering, what about my sports car? I don't want it to be a boring wagon. I've got good news.
Say hello to the shooting brake.
At some point of course the SUV craze will still happen. But that's easy to handle.
Just take your wagon, jack it up a few inches, and add some plastic cladding. No need for automakers to kill off treasured nameplates.
So now I've made the cars of the 20th century far more space efficient as it always should've been.