There is no amount of improvement in an ICE that can get it to be even HALF as efficient as an electric motor. The best electric motors are about 94% efficient, and I very much doubt that any ICE (or ECE for that matter) will ever exceed 47% efficiency peak; let alone average above 42%.
So I think that all efficient cars in the future will have battery packs and plugs. The only question in my mind is whether they also have an engine, and how they use that engine i.e. in a serial or in a parallel mode.
Before you get all excited, you have to think about how an engine works best: and that is at a narrow range of RPM; and in "bursts". All those MPG contests when the cars get >1,000MPG only have a very small engine and they only run the engine in short bursts and at a very specific RPM. These cars are hardly practical in the real world.
Contrast this with the
SolarWorld GT car built by Bochum University, which is relatively practical and very efficient -- it only uses solar PV panels on the car, and it is driving around the world.
Another example is the Edison2 VLC: the ICE version got 110MPGe on the EPA Combined test -- which is fabulous for a 4 seat car. But the electric version got ~245MPGe, and an AC electric drivetrain probably would be over 300MPGe, and it would be far quieter and a lot quicker, and smoother, etc.