The new BMW M3 will, like the new M4 Coupe, get a lot more aggressive style. The M3 sedan will get the same body kit as the M4, with a new front- and rearbumper. Bigger wheelarches and a roof from carbon-fibre are also one of the many differences between the M3 and the normal 3 series. I’ve made a rendering a few months ago of the BMW M3 too. You can find that one here.
The new BMW M3 and M4 will lose there 4.0 V8, because of the Co2 emissions. Likely, it will be replaced by a twinscroll turbocharged 3.0 inline-6 with more horsepower’s than the current M3. Something in the region of 430 to 440 bhp is likely, and transmissions will include BMW’s 7-speed dual-clutch gearbox or a 6-speed manual. Overall weight should be lower than before, thanks to more extensive use of aluminum and carbon-fiber components.