This could get very interesting.
A 1.6l BMW diesel (perhaps 2.0l for larger vehicles) is more than adequate to maintain a top speed, particularly if optimised for a peak power output.
Electric motors can easily provide adequate acceleration power equivalent to even a 3.5l engine.
Li Ion batteries could easily provide energy to run for 5 minutes at, say, 80mph without excessive weight penalty.
The latest battery technology will accept charge at very high rates.
BMW have already developed energy recovery during braking and stop-start.
Put these together and you have a diesel electric system where the diesel only runs for a short time at optimal efficiency thus minimal consumption, the range is even better than a standard diesel, the power (when needed) is equivalent to the best gas or diesel.