My ideal DMU would look a bit like a Blue Pullman.
Nominally 7 coaches, but flexible.
Vehicles 1 and 7 would have a driving cab, an engine compartment and a small passenger saloon. The 2 power plants would need to be sufficient to shift an 8 coach train.
Vehicles 2 and 6 would be passenger saloons (no engines) but with power bogies, semi-permanently coupled up to 1 and 7.
Vehicles 3-5 would be intermediate trailers, this is where the flexibility comes in, you could knock one out or put an extra one in. One option could have a mini buffet.
Note that with this layout, 1 and 7 would only have passenger doors at the inner ends, so a 7 coach set would fit a 6 coach length platform.