I LOVE THE i3! That would be such a great base model. As long as Apple don't make this car literally cost an arm and a leg...and your first born.
It would make sense that Apple would choose a Luxury brand as a base, however there's several issues (mostly specific to North America)
1) Not fully electric - the i3 has a model with and without the gas engine, I'd assume the Apple vehicle would be without the gas engine. I could see Apple trying to address the battery range/charging speed issue much in the way they've tried to address it in the iPhone/iPad.
2) Likely unusable in North America outside of LA/SF/NYC/SEA - If you've ever tried to use Onstar or GPS systems, or "cloud driven" maps like Nokia, Google Maps or Apple Maps 20 miles outside of a major city, you'd find them to be useless due to the lack of usable cellular service. The situation is far worse in the Pacific Northwest parts of Canada and the US, but the prairie areas (Montana, North and South Dakota, Saskatchewan, etc) tend to have complete dead zones.
3) If it's autonomous, it will only be available in select cities that have 100% LTE coverage, including inside parking garages of shopping/hotel/convention facilities. So far batting zero.
4) Plastic cars = death trap. The increase in fuel economies in cars over the last 4 decades has come from switching from Steel to lighter materials. The end result is that a collision with just about anything is fatal, especially commercial trucks and trains. And cities keep building non-automated light rail systems with level crossings, which only increases the danger.
I remember 20 years ago that two sticking points that dealers kept mentioning in the area I was in was:
1) Never buy the Asian cars - They can't deal with road salt, so you'll be junking it in 5 years, they need specific coatings to deal with Canadian climates*.
2) Never buy the two smallest(also cheapest) car models - As they are not solid vehicles (made of a lot of plastic and paper-thin metal) and if you are in an accident with one, they will bury you in it.
* Which is strange considering that Japan/Korea and southern BC/Washington's climate is pretty much equal. Any car designed and built for California would actually not have appropriate winter materials for the same climates, which is a concern for battery-electric cars.