I believe you can already tie a complex series of rail movements to a single logic circuit, then activate that circuit from a remote button when in Flight Mode with a single press. You could do this to begin a walking or rolling sequence no problem.
However, making wheels and legs work would require changes to the collision detection system to take into account friction between surfaces. Also, you'd have to consider in your design how your vehicle would balance and remain upright in gravity. Especially tricky for vehicles with legs! Not a bad idea, but would require more "ground-work" (pun intended) than simply making Planets big enough to make such vehicles worth-while.
Everything else you mentioned also sounds good but, yeah, is probably best discussed in other threads
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Actually, I'm amending my idea. Adding 1 hover block to your vehicle will allow it to hover at a default height, say 1 meter above ground. Your vehicle can then move in 2 dimensions like the Player does in gravity. However, unlike Player movement (which instantly reaches maximum speed), your vehicle will need to accelerate like a ship does. The max speed (just like the current Thruster speed limit) is a server default, say 25 m/s. The more hover blocks you have, the greater your rate of acceleration until you reach the max speed.
In addition, you should be able to hold down the jump button and it will accelerate you
upwards. The rate you ascend is also determined by the vehicles mass/hover ratio. Your ascent will continue until you either release the jump button or you reach the edge of the gravity field.
This would allow heavy vehicles such as Tanks to make small, slow "jumps" to clear awkward terrain, and lighter "flying" vehicles such as helicopter gunships to remain hovering at higher altitudes with relative ease.
The only problem I can see with this idea is: what happens if you put both Hover jets
and Thrusters on your vehicle? How does the game know which type of drive should be used?
What does anyone think?