I'm confident the M2 will be a 6 cylinder. It has to be a step up from the M135i or at least share the same platform. It wouldn't surprise me if they reused the same N55 engine from the M135i with a different tune and give it the usual M treatment as they have done for the E82 platform. Main differences will be widebody, LSD, M suspension, quad exhaust and M badge. This on its own merits justifies its value, imo.
Current: Mercedes CLA45 AMG, Porsche Macan S
Past: BMW 135i, BMW E46, Ninja 300 ABS