As naturally aspirated gasoline engines have gotten more and more powerful for their size, it's inevitable that smaller engine displacements will be used to improve fuel economy. I don't own one yet, but from reports it sounds like the ProMaster is almost there with the 3.6L V6.
Since this engine can produce close to 300 HP, which is far more than normally needed for cruising, engineers then have to gear the vehicle extremely high so it gets decent fuel economy on the highway. The problem with this is that engines operate at too low an RPM during slower cruise. And this can also add to excessive gear hunting.
One possible trade off is using a smaller engine with commensurate lower gearing so it would operate at higher RPM much like vans of decades ago. As an example only (to make math simple to follow), let's say a PM with 3.6L V6 runs at 2,000 RPM at 65 MPH.
If the same van had 4/6 of the engine, or a 2.4L I-4, it would need to run at 3,000 RPM at same 65 MPH in order to have same BMEP, which is required to maintain equal fuel efficiency.
A smaller engine would obviously have less max power (although same as diesel), but by operating at 3,000 RPM instead of 2,000 RPM at cruise, the torque curve would be at a flatter point and closer to maximum, rather than the V6 being way below its max torque RPM. So if the vehicle slows from 65 to 50 or worse to 40 MPH, the V6 would be just above idle forcing a downshift. The smaller engine with much lower gearing would still be in the 2,000 RPM range or higher, so it could hold top gear easier.
The biggest disadvantage with the existing 6-speed transmission with its wide ratios is that if a hill at 65 MPH forced a downshift, RPM would jump from 3,000 to around 4,500 instead of from 2,000 to 3,000. Most owners wouldn't like that.
So an engine in the middle of these two examples, around 3.0L but with a 9-speed transmission with close ratios, might be OK as long as final drive ratio is made much lower.
For me personally, I'd drive at 60 to 65 MPH so a 2.4L I4 with a 9-speed would be great as long as the final ratio is low enough -- like around 5:1.