Well for one thing the mass of the flywheel is what it is because manufacturers have to strike a balance between performance and drive-ability and emissions figures. A lighter flywheel would require the car to idle at higher RPM's to keep from stalling because the inertia of the flywheel is what keeps the engine firing when you're not on the gas.
So yes, it can fairly easily be done, but the reason it's not is because most consumers will not be happy with a car that idles at 1000RPM and the relevant country's ministry of transport/EPA/whatever won't like all the polarbears you're killing.
As for dual-mass flywheels. They're great and all, but they're hellishly expensive to replace WHEN (not if) they go bad. E.g. my Subaru is pretty old school with its single mass flywheel, but it would cost a third to replace it compared to the dual-mass setup in the equivalent Mazda.