[E90 Skye]

this will be hard to do as what drives up the weight in cars these days is not the engine or the construction materials or the process they employee. its mostly bc of government and automotive regulations that weight goes up. safety features, and more technology in comfort and passenger protection.
Think if we didn't have motorized leather heated/cooled seats? those things weigh a ton on their own. So i don't see weight going down significantly, but there could be methods where we can maintain the current weight or cut it down just a bit. Until carbon fiber production becomes cheaper there won't be cheap solutions to this.
The current M3 motor is lighter than the 3.2L i6 it replaced but the car is heavier still. (partially due to the fact that they made it bigger to please the market need for bigger cars and partially b/c of added safety and comfort features).