Developing a diesel hybrid would be an inefficient use of resources.
In terms of fuel savings, let’s say making the Prius into a diesel hybrid lets you increase from 45 MPG to 70 MPG. That will save you roughly 79 gallons per 10,000 miles. However, increasing fuel economy from 12 MPG to 15 MPG in a larger vehicle will save you about 167 gallons per 10,000 miles. So the focus should not be on increasing fuel economy in vehicles that are already relatively efficient. Much more fuel will be saved by making large vehicles run more efficiently.