Not just direct upgrades that you switch in place, but actual choices how the vehicle functions.

MechWarrior games are good examples as there you have a top mass for the walkers to which you need to fit all your armor, weapons, ammo, reactors (top speed), and any other special equipment you may want.

The emphasis here is on customization, not upgrades. Although you obviously want upgrades as well, but these have choices over what aspects of the vehicle's performance you want to improve instead of having some specific part that's better than any of the other choices simply because it improves all aspects as much as a specialized engine does for single aspect.

Customization limited only to weapon choices should not usually be included.