I am still slightly surprised Tesla didn't set themselves up as a supplier to all other manufacturers and dominate EV powertrains. They seem to have powertrains working quite well, it's the rest of the vehicle that takes time to learn how to design from scratch. My guess is EM needed his ego stroked by having full design control and his company logo on the cars.
They tried this. IIRC Tesla supplied some powertrain components to Toyota and Mercedes-Benz. That was for low-volume products that didn't play into Elon Musk's vision.
Problem here is that, at least in the opinion of the rest of the auto industry, Tesla doesn't know how to do anything that they don't, so why pay Tesla for something that (at least in theory) you already know how to do?
I’m really surprised that the Model 3 is a sedan and not a hatch. They could have maintained identical body lines and made it a hatch with fold down seats that would offer a cavernous amount of cargo room in the back.
Swing and a miss.
This has come up before. Evidently they were very fussy about the roofline for aero reasons. Having it a hatch would necessitate a structural cross-bar high on the rear window which was getting into rear seat headroom. Rear seat headroom is supposedly why the roof and rear window are one enormous piece of glass.
Not saying I agree with the design decision, but that's what was stated.
IMO what Tesla should have done, was contract with an established auto industry supplier (e.g. Magna) to do the entire bodyshell including interior, HVAC, and all the stuff to make it all work, and kept what they are good at (electronics) internally. Magna is quite capable of building this entire vehicle to normally-expected auto industry quality standards. No doubt Magna would have forced some design changes but I'm sure they would have been for the better.
Random stuff that it is KNOWN could have used some design tweaks: Trunk latch too hard to close. Trunk over-counterbalanced. Trunk seal doesn't catch water rolling down the rear window. Too hard to get to main battery disconnect following a collision (relies on either a functional touch screen or a non-destroyed front bumper AND is reliant on vehicle electrics being awake, which is not guaranteed after a collision). No mechanical inside latch to allow rear passenger doors to be opened from inside if electrical system is down. There is a ton of other stuff from the manufacturing side - stuff that could have been easier to manufacture if specific design and/or process changes had been made (and no one would have known the difference, except for the car perhaps being a fair bit lighter).
Tesla knows batteries and electronics. The "car stuff" ... not so much.