RSG: Ship Design
Players have a "Ship Creator" tool to customize the look and functionality of their ships.
Modules makes the ship creator tool much simpler to use, and gives the game a need for some kind of currency.
Salvageable modules and parts mean that provoking engagements and pirating become an aspect of gameplay - also salvaging wrecked ships provides an alternative career path.
Not necessarily a separate module, but the locations of hard points of the guns should be variable.
Mining lasers are the primary tool of the space pilot, for use as weapons, for mining, and scavenging. Lasers are not slow, like Star Wars "laser bolts" but more instantaneous.
Other weapons like rockets, mines, and EMP's can be bought and scavenged for use.
Engine, Fuel and Cockpit modules are required for the smallest ship, and as the ships grow, the need for crew quarters and storage holds becomes necessary.
Perhaps a bonus for having spinning sections that give centrifugal artificial gravity.
Weapons (missiles, lasers, kinetic weapons), extra fuel tanks, solar panels, power supplies, mining equipment, holds for storing the product of your work, empty modules put there to make your ship look more formidable than it really is, escape pods, long range radar & tracking, robotic command module, defensive devices such as flares and chaff dispensers (to fool heat and radar-guided missiles), ablative shield (not 'force-fields' but large chunks of some kind of plastic or ceramic that the enemy has to boil away with his lasers before he can get to the juicy targets), crew quarters, passenger quarters, luxury quarters, backup command module, solar sail, medical bay, bulk cold storage facility, water tanks, grapples for salvage work, spare parts, airlock, small 'shuttle-craft', prison facility, extended life-support facility.