Almost 10 years ago I blogged about car sharing companies in Melbourne . Since that time the use of such services appears to have slowly grown (judging by the slow growth in the reserved parking spots for such cars). This isn’t the sudden growth that public transport advocates and the operators of those companies hoped for, but it is still positive. I have just watched the documentary The Human Scale  (which I highly recommend) about the way that cities are designed for cars rather than for people.
I think that it is necessary to make cities more suited to the needs of people and that car share and car hire companies are an important part of converting from a car based city to a human based city. As this sort of change happens the share cars will be an increasing portion of the new car sales and car companies will have to design cars to better suit shared use.
Luxury car brands like Mercedes support storing the preferred seat position for each driver, once the basic step of maintaining separate driver profiles is done it’s an easy second step to have them accessed over the Internet and also store settings like preferred radio stations, Bluetooth connection profiles, etc. For a car share company it wouldn’t be particularly difficult to extrapolate settings based on previous use, EG knowing that I’m tall and using the default settings for a tall person every time I get in a shared car that I haven’t driven before. Having Bluetooth connections follow the user would mean having one slave address per customer instead of the current practice of one per car, the addressing is 48bit so this shouldn’t be a problem.
Most people accumulate many items in their car, some they don’t need, but many are needed. Some of the things in my car are change for parking meters, sunscreen, tools, and tissues. Car share companies have deals with councils for reserved parking spaces so it wouldn’t be difficult for them to have a deal for paying for parking and billing the driver thus removing the need for change (and the risk of a car window being smashed by some desperate person who wants to steal a few dollars). Sunscreen is a common enough item in Australia that a car share company might just provide it as a perk of using a shared car.
Most people have items like tools, a water bottle, and spare clothes that can’t be shared which tend to end up distributed in various storage locations. The solution to this might be to have a fixed size storage area, maybe based on some common storage item like a milk crate. Then everyone who is a frequent user of shared cars could buy a container designed to fit that space which is divided in a similar manner to a Bento box to contain whatever they need to carry.
There is a lot of research into having computers observing the operation of a car and warning the driver or even automatically applying the brakes to avoid a crash. For shared cars this is more important as drivers won’t necessarily have a feel for the car and can’t be expected to drive as well.
Generally cars are designed to have 2 people (sports car, Smart car, van/ute/light-truck), 4/5 people (most cars), or 6-8 people (people movers). These configurations are based on what most people are able to use all the time. Most car travel involves only one adult. Most journeys appear to have no passengers or only children being driven around by a single adult.
Cars are designed for what people can drive all the time rather than what would best suit their needs most of the time. Almost no-one is going to buy a personal car that can only take one person even though most people who drive will be on their own for most journeys. Most people will occasionally need to take passengers and that occasional need will outweigh the additional costs in buying and fueling a car with the extra passenger space.
I expect that when car share companies get a larger market they will have several vehicles in the same location to allow users to choose which to drive. If such a choice is available then I think that many people would sometimes choose a vehicle with no space for passengers but extra space for cargo and/or being smaller and easier to park.
For the common case of one adult driving small children the front passenger seat can’t be used due to the risk of airbags killing small kids. A car with storage space instead of a front passenger seat would be more useful in that situation.
Some of these possible design choices can also be after-market modifications. I know someone who removed the rear row of seats from a people-mover to store the equipment for his work. That gave a vehicle with plenty of space for his equipment while also having a row of seats for his kids. If he was using shared vehicles he might have chosen to use either a vehicle well suited to cargo (a small van or ute) or a regular car for transporting his kids. It could be that there’s an untapped demand for ~4 people in a car along with cargo so a car share company could remove the back row of seats from people movers to cater to that.