The car obviously will be a hybrid which is why the collaboration happened in the first place. Toyota has some great electric motors and hybrid technology which they have experience with from the Prius and BMW does as well with the 'i' brand now. We will see some mixing and matching as BMW's lightweight technology is paired with Toyota's electronic designs that BMW will produce.
The expected result is a lightweight 3085 pound car. This is a great weight target for a sports car and especially one with a hybrid drivetrain. Power is expected to come from a direct injected 2.0 liter four-cylinder motor. Let's hope that the motor is BMW's as the N20 is a very strong candidate.
This motor of course will be paired with an electric motor or two and the output should be in the 350+ horsepower range. That sounds pretty good with the 3085 pound curb weight making for a car that is fun to drive and toss into the corners.
The car may not be rear wheel drive as the electric motor will be front mounted. That means the next generation Z4 and Supra likely will have all wheel drive.
So what is the target date for these two cars? 2017. The end result is going to be very interesting to see.