Is it out of the question for BMW to actually do something unique and allow a 320hp 235i to exist at say 3,300 pounds (1,500kg) and an M2 with an N20 with around 306hp and 3,100 pounds (1,400kg)?
Lighter weight through use of carbon, aluminum and deleting some fluff like insulation and the silly carpeted rear fender liners could allow them to do this. While you'd need to add a limited slip, you could leave the brakes and drivetrain alone, as they'd be dealing with less weight.
I suppose the real challenge would be convincing paper jockeys that a less powerful car could be better than a more powerful one. I won't hold my breath for this, but would be in line for one if they went this route.