This is more like storage on coaches but on a much smaller scale, and particularly useful for messy (kids') stuff.

Looking underneath my mpv I see a lot of empty space alongside the fuel tank, etc.

A decent sized storage compartment could fit down there without altering the shape or size of the car. It wouldn't come any lower than existing fixings so ground clearance would be the same and the car's aerodynamics are unlikely to suffer (and it is an mpv after all).

The compartment door be a small, fold-down type, located just under the rear kerbside passenger door. Maybe there could be one compartment on each side of the car, mechanical gubbins permitting.

It would be shaped to perfectly fit a pushchair and ideally located to place muddy, smelly shoes, boots, scruffy towels from a day at the beach - anything you don't want to take inside the actual car.

A full hard-plastic interior would mean it could be hosed out as need be.

It would probably be just the right size for a small child too on those more stressful days out.