I doubt there will be any ICE efficiency breakthroughs that will amount to anything.
As of now, there have been a gasoline car prototype that got 120mpg (Aptera) and a production diesel car that gets 240mpg (VW XL1). The $30,000 Toyota Prius Prime gets +50mpg. Technologies, like free pistons, ceramic engine materials, and filtering out the nitrogen so that compression ignition, lean burn, stratisfied charge and high compression technologies and be used to their full potential without the production of NOx emissions do exist, although costly.
So the breakthroughs are out there. And 300mpg cars are possible. But the problem always seems to be that in order to get better efficiency the price goes up considerably. It's cheaper to buy and own a non-hybrid, like a Toyota Camry, that gets a lot worse fuel mileage than a car like a the ones mentioned above, even the Prius. If you're pinching pennies it doesn't make much sense to spend $30,000 on a car that will only save you $3,000 in fuel in the next five years compared to a $15,000 or $20,000, or even $25,000 car. Add to that the added maintenance and repair costs of the more efficient yet more complex vehicle.
For this reason, the majority of "breakthroughs" aren't true breakthroughs, but simple tuning techniques. With all due respect the ICE really hasn't changed all that much over the past couple centuries. They mix fuel and air, compress it with a piston and ignite it with a spark plug. Yes, fuel injection is better than carburetion. Yes, there have been improvements. But there's nothing that's really all that great about modern day ICE technology except the exuberant price.
When you take the ICE out of the equation then EV technology becomes quite competitive. EV technology keeps getting less expensive, yet ICE technology keeps getting more expensive, outweighing any improvements in fuel mileage. And EV technology is generally much less complex and much more reliable. Really the only hurdle is the battery life, and solving that in something as simple as the right chemistry and you might just have a very reliable, simple, cheap and efficient vehicle that will appeal to the masses.
Actually the main reason why I got my Leaf is because it was the only car on the market that I could afford that got better fuel mileage than my 1985 VW diesel. And sadly a lot of people still think 30mpg in a small car is awesome fuel mileage.
2013 SL SOLD
1972 VW Beetle SOLD
1984 VW Golf diesel CURRENT
2013 Toyota Avalon Hybrid CONTEMPLATING