Weight increase has more to do with regulations than the engine. Higher crash standard dictates more metal in the structure which increase weight. Increase weight from that means a more powerful engine and a V8 has better chance than a I6. And in reality the current V8 engine by itself in M3 isn't that much heavier than the twin-turbo I6 in a 335i.

What BMW needs is to explore composite materials, like plastic and carbon fiber for hoods, trunk and fenders, light weight carbon shell seats, thinner glass or plastic glass, etc. They are already onto something with the carbon roof. A full carbon dash will weight a fraction of the current leather one, carbon door panels same thing, carbon center console, etc. Forged magnesium wheels will weight a bit less than aluminum ones too, carbon brakes less than steel ones, carbon suspensions over aluminum ones. Many many ways to reduce weight, but at what cost?

But then again these exotic material will raise the cost of the car and price it out of range for normal people.