Fun until you get all the upgrades. there needs to be a harder setting than "Heavy Enemy Presence"
Maybe there could be another level of ship, too, like a space station, and it would be way tougher than a mothership. There should be higher upgrade maxes, and more kinds of upgrades. for instance, mining speed, missile explosion damage/radius, blaster regen rate, etc.
Since you have a decent A.I. program for drones, you could have purchasable helpers! You seem to have been planning something like that, when you kill yourself, it says "Friendly unit destroyed!" if you decide to further develope this game, maybe add a multiplayer function/version, this could be BIG.