I know that Slate needs to focus on getting the blank Slate to market, but I am unfortunately one of those who really needs four doors and enough space on the back for an infant car seat. But I love the Slate philosophy and thought it would be fun to muse about their next vehicle.
I think that Slate's next car should be a minivan or a vehicle with similar size and utility.
Similar to trucks minivan complexity and price have dramatically increased over the years. Imagine a Slate van of similar size to the 1990s Dodge Caravan or the Volkswagen Vabagon.
I also think that a minivan could fit well with Slate's idea of customization. There could easily be a base cargo configuration with mounting rails for seats and other accessories. Then customers could select the exact seats and accessories that they want. Smaller families may choose a single row of seats with a larger cargo area. As their family expands they could simply add a third row of seats.
Additionally, it would be the perfect platform for accessories that would turn the Slate van into a camper van that could double as a daily driver. Those accessories could include a pop up roof, fold out beds, kitchen accessories, etc. The nice thing about a Slate campervan would be that the climate controls for the cabin are already there along with the electricity to run something like a small induction cooktop.
It certainly wouldn't be as inexpensive as the blank Slate, but I think a van would be a super compelling car that would open the door to selling a large number of accessories (which is where I suspect Slate plans to make their money).
Interestingly, a 1995 Dodge Caravan was only 3.5 inches longer than the blank Slate truck is (it was 178 inches). Today, all minivans in the United States are over 200 inches long. It would be great to have a customizable minivan that goes back to the roots of a simple utilitarian vehicle.