This dream is now possible with garry's mod !
Even if it's old, it's still a pleasure to play this "game" : Built on the source engine, it let the user make everything he wants from a simple boat with some wooden plate and thrusters to a complex helicopter based on the control theory.
Moreover there is a lot of mods like
- Fort wars where you have to build a fort to protect yourself while you keep the ball in the fort
- Flood mode where you have to survive against enemies when all you have is weapons and a boat
- Race : You build your own car and fight against other players !
- RP : play role play in a city (not my favourite)
- Maybe the best mode : Sandbox : build what you want !
Here are an example of the thing you can make in the sandbox : by placing baloons on something you can make a ballooning.
With some plates and elastics you can make a catapult.
With a sensors and a turret you can design your own killing machine !
And a lot more...