well as in earth 2160, all units have a basic menu button like normal. it builds the unit with the default features and if you want to customize later in the game, you can go to a menu to customize and add the custom unit to the build menu. this makes the feature usable because theres no point in having the customize window open when you havent researched the addons that require it. adding or subtracting wheels, changing the type of wheels or treads, power plant, WEAPON, turret type, chassis, etc. theres actually alot that can be done with such a system. it could be made as complex or as simple as we want. in earth 2160, you can even change the complexity of the window. you can set it so that you scroll thru short lists of addons categorized as land or air targeting, fast, powerful or default variants. it can be turned up to expert mode to allow you to scroll thru long lists of every type of turret, engine, power plant available.