Ship upgrades.

Firstly, the player will be able to buy new spaceships. These MUST be visually different in some small way so as to give feedback to the player. The different ships have different sizes and masses (bigger means you can fit more upgrades).

As for the upgrades, here are my initial ideas. Of course these will be subject to play-testing before they go in the game.

Extra fuel tank - gives more fuel.
Stronger engines - gives more thrust.
Inertial damper, rotational - allows player to stop ship rotation.
Inertial damper, movement - stops the craft.
More powerful guns (of course)
Homing missile.
Shield (and upgrades of).
Chaff (to throw off enemy homing missiles).
Tractor beam.