How about a fast car while the tech demo is still running? A buggy, a F1 racing car, a modified train, or a slider controlling the properties of the already existing truck. Whatever. Just something fast.
I would love a fast car, if not for testing if the engine can keep up, then for just playing around. The tech version would be so much more fun to play around in with the ability to roam around on the dunes, slide down cliffs and speed down hills.
I think all the current vehicles / planes are too slow compared to realism. And with a world size like this, running out of movement space is not a problem. I don't see why we should not have any fast vehicles.
PS: Is it normal for the truck to fall through world when landing upside-down?