Here are some of my current ideas:
1. The second row definitely needs better A/C airflow. The third (currently trunk) has none at all. And I'm not fond at all of the front air vents either... they don't have a whole lot of flexibility to direct air where I want it, and they feel cheap. Overall, MUCH better A/C airflow and design is necessary throughout the whole car.
2. At least one cupholder per occupant.
3. Some storage per occupant. At least door pockets and seatback pockets are mandatory, as is a fold-down center section of the rear seat for additional storage/cupholders/armrests for the two rear-seat occupants on the sides. The complete lack of storage is a MAJOR downside from my wife's point of view, and she's tempted to forgo the X and keep her BMW because of that. :-(
4. Some way to do rear-seat entertainment. I'm not fond of proprietary RES systems because they're very limited and they become obsolete very quickly... but kids get tired of holding up a tablet, OK? Either a way to have a built-in RES, or perhaps a seat-back bracket to hold a tablet, or SOMETHING that lets the kids watch something handsfree.
5. I'd really, REALLY like to see a mobile app that I can authorize from the front touchscreen. That way, authorized users could control the A/C and music settings from their phone/tablet, and they could see the navigation/energy progress from the apps. Plenty of cool stuff you could do with this... but just letting the kids in the back control the car's music through the app would be a godsend.
6. Please, the built-in WiFi hotspot for everyone. If necessary, offer it as an additional purchase so the car so the car can have a mobile-data plan for firmware updates and whatever Tesla needs, and a second mobile-data modem that can power the hotspot. The hotspot should ideally be modular, as should the mobile-data modem, so that we can upgrade from 802.11n to 802.11ac or from 3G to LTE as technology progresses.
7. The ability to install an SSD in the glove box, or at least another USB port, for music storage so I don't have to have a USB memory stick always using one of the ports between the seats.
8. As both a safety and comfort feature, the car should sense if any seat is occupied (weight plus heat?) and not turn off the power -- especially the climate control and music -- if the driver steps out. If we're going somewhere and I forgot my hat, then I have to put the car in neutral and set the parking brake while I go back into the house. If I just put it into park and step out, my wife and kids are left in a dead car with no A/C and no tunes. Does not buy goodwill.
9. A top-down 360-degree view for parking. My wife's BMW has this and she values it immensely.
10. Side sunshades for the rear windows. Manual is fine, electric is ideal. Again, my wife's BMW has this and she loves it.
11. Some towing capacity with a standard/removable rear hitch, which I could also use for a bike rack.
12. The ability to REALLY raise the car for much better ground clearance, whether that's for rough roads or muddy/snowy conditions.
13. Absolutely mandatory, at least one powered USB port per occupant, with at least 10W (2A @ 5V) charging capacity.
14. Better range for the key fob sensors. My S often doesn't know I'm there unless I approach from the front or unless I'm very close.
15. A desktop app that allows you to route/plan as you would if you were in the car.
16. The ability to send an address or routing to the car's nav system.
17. The ability to set an "exit/entry" profile. Suspension, seat position, wheel position, and so on.