Several advances are already coming along well, but not in the mainstream as most car makers don't want people saving fuel.
1) improve aerodynamics;
2) improve engine efficiency.
I've done some of this to my '99 honda, am getting 55 mpg on trips and aiming for 60. Also, I don't drive very much.
Engine efficiency can be greatly improved with driver technique, some of which can be ingrained in cars when they're made. The 2014 honda fit is a good example of this, with dual electric and gas engines. However, it still only gets 55 mph. IMHO, most cars should be in the range of 60 mph and some should be able to get more than 100 mph. Modified cars from the 1990's are already getting in this range.
Hybrid cars like the fit turn off the engines at stops, and easily start them again, without turning the key off and back on, i.e. no difference than a regular car for the driver. This city driving can get as much or even better mileage than on trips.