This is a great start and a fun little time killer. In addition to some of the other suggestions I see, I think adding some kind of unique features or different materials as you progress through zones would help encourage people to play more. Something like planets with combinations of materials (like a planet with 75:25 metal:gold, or a new set of mats that could be used toward new upgrades)
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Interesting idea. We will definitely add something like this to keep things fresh :)