The thing to always remember when reading these new ideas is that they generally do not create energy, only store it. And the source from which it comes to them, the source from which they draw the energy they store, is the gasoline. Even the flywheel in only storing energy that the engine (gas) created in the first place. And there is always a loss in converting energy. In the case of the flywheel, the loss would be bearing friction and heat loss. A flywheel will return much of its inertial energy to the vehicle, but cannot return more than it acquired from the gas burned to get it spinning.
The ideas that conserve energy, like those that reduce weight, drag, or combustion losses, are the ones that interest me. These include improved manufacturing technologies (including casting), better materials technologies such as polymers to replace metals, technologies to improve the fuel's combustability (like direct injection), and even things to reduce losses due to reciprocating masses (like lighter reciprocating parts, and even a rotating ball-valve system that I saw once to replace valvetrains).