Changing subjects a little, with Z-levels coming out SoonTM are there immediate plans to include flying vehicles?
It immediately occurs to me that there are two options for flight, and two methods for each.
1: Speed-related flight, namely propellers and jets.
2: Sustained gliding, namely wings and gliding frames.
3: Active hovering, namely helicopters and and VTOL.
4: Lighter-Than-Air, namely hot-air balloons and blimps.
These would ideally be able to interact with wheels without clashing, so you could for instance install VTOL jets on a tank chassis and make limited lifts, or blimp balloons on a quad bike so you could lift from danger a bit, or - most importantly - rockets on a motorcycle so you can fly over buildings.
Vehicles are already aware of momentum, if you're traveling west then it continues coasting west. This would be important for gliding vehicles, as it should be possible to, for instance, drive a motorocycle forward to gain speed, activate a VTOL jet/rotor to raise into the air, and then use existing momentum with gliding wings to stay aloft while slowly falling and gliding forward. This would be the most complicated scenario, combining wheels, VTOL, and gliding.
Other combined motors would be relatively easy. When on the ground, the truck uses regular V6 engines and wheels and whatnot. When activating flight mode (any method to lift off the surface) then it should stop calculating the ground engines and stop consuming fuel, and start calculating the air engines and wings. So the truck could be equipped with compressed air tanks and a blimp frame, inflate the balloon, and then use a small propeller to navigate over a river or thick forest before settling down again and turning back to wheels.
The REAL nice addition I'd like to see would be shifting power. Being able to turn engines on/off, and being able to turn a V8 to power the wheels, then change it and use the SAME engine to power a helicopter rotor would allow for some really fun types of designs and hybrid vehicles.
As a footnote, we should also add a "hovering" tag, which would allow a vehicle to 'fly' at ground level, high enough to ignore piles of rubble and traps, but still low enough to collide with zombies and walls.
ALSO what about amphibious vehicles and boats or submarines? With Z-levels available, deep rivers could have an under-layer, allowing creative use of submarines, boats, or even just trucks that have a really tall profile and are water-tight and just drive across the riverbed.