I still think it would be great to have a diesel engine hybrid with a Chevy Volt-like propulsion system, perhaps with even smaller batteries than the Volt. The engine could run at peak efficiency and let the electric motors provide motive power.
Diesel engine + all of its associated emission control systems + hybrid system components = too much money.
Or, you recognise that since the way the vehicle drives no longer depends much on the shape of the torque curve ... you take the gasoline engine and fiddle with the cam timing to obtain Atkinson-cycle operation, which results in little low-end torque but it doesn't matter because the electric motor operates best there. See: Toyota Hybrid Synergy Drive.
There are limits to the concept of "run the engine at peak efficiency and let the electric motors provide motive power". The problem with that, is that the power available close to the engine's sweet spot of highest-efficiency operation rarely coincides with road load, with the result that the engine will always be charging or discharging the battery, so you get charge/discharge losses. That's why the Toyota Prius sends most of the engine torque through the mechanical torque path, and it varies engine output to match road load up to certain limits. That's why a Volt is capable of mechanically coupling the engine to the wheels, and it varies engine output to match road load up to certain limits.
It's worth operating the engine "off design" sacrificing a few points of efficiency if it allows NOT having the charge/discharge losses.
The way the Priuses and Volts and Ford hybrids all work when they are in combustion-engine mode, is that they try to operate the engine within its best-efficiency regime, matching road load, driving the wheels mechanically, whenever they can. If there is so little demand that engine efficiency would be poor, and the battery is sufficiently charged, they shut the engine down and operate electric-only. If the battery charge drops too low then they start the engine and run it under load to recharge it - the idea being that the total of the road load and the recharging load gets it back into its best-efficiency regime.
It is quite a sophisticated optimisation strategy ... But the driver still plays into it. I've had a Ford Fusion hybrid as a rental car, and the trick was to play along with the state of charge of the batteries, taking advantage of terrain where possible, the idea being to help it to run the engine as little as possible but IF it was running, keep it under load.