Sure, they can do better. The question is, would you buy it?

In 4x4 trim, the Ecoboost is rated at 15 city/21 highway. That's equal to or better than all their mainstream competitors.

The only one that does better is Chevy/GMC with the hybrid Silverado/Sierra. It's rated at 20 city/23 highway (4x4), which is the best of any truck currently on sale in the USA.

However, the hybrid system adds about $7,000 to the price of the truck, it adds additional batteries that need to be replaced every ~10 years or so, and there is a significant reduction in payload and towing capacity. Towing goes from 9500lbs to 5900lbs. Payload goes from 1712lbs to 1509lbs.

For most people, the slight increase in fuel economy isn't worth the increase in cost and decrease in capability. This is why sales of the Silverado Hybrid are simply miserable.

Along the same lines, Dodge has been talking about adding a small diesel engine to their half-ton Ram trucks. That would easily net the 18+ city/25+ highway you're looking for. However, the engine would be at least a $3,000 option over the 5.7L Hemi, it would be significantly less powerful, it will require Diesel Exhaust Fluid, and diesel fuel itself is presently more expensive than gasoline in most areas.

Again, it's all a big gamble as to whether or not people will actually buy it. Ford took a big gamble with the ecoboost, as they weren't sure if people would pay more for a smaller and more complicated engine, just to get a few better mpg than competitor's top V8's. So far they've been lucky, as the Ecoboost has been selling extremely well.