Add-on which allows you to take certain bodies and parts from vehicles and create your own. Flying bodies/wings/ability would be obvious. Weapon, wheel, wing, and body customization. This is within a "window" so its not within events.
Same for weapons. Small weapon customizer. For dem create a class TDMs. Customizable body, barrel, stalk, and whatnot. Machine gun, pistol, whatevar.
Also... This is all from a list of options; many limitations, but customizability.