I have a very simple suggestion. Add distance and time. Time makes the game hard because you can compete with yourself and get a better time. Distance is for letting your player know when they pass different stuff in the universe. Like leaving the milky way for example. Then I would play this game over and over in oder to beat the best time to Jupiter or something like that.
What this game lacks is some kind of mesurement for success.
Now all you do is fly over and over again to gain money to buy better ship to get higher up. But why? Have I gotten any better at this?? I do not know and that is the problem and why the game feels grindy.
Make the game about reaching cristmas before christmas so you acually can fail. Without the possibility of failure there is no possibility of victory either.
For those of you who complains about the real game costing money or the free game is too short. This might be intetional. Perhaps the maker of the game wanted some money for all his hard work so that he would be able to continue with game development.
More strategy is needed. Like this it feels simple. There are not many different ways to play this game. The best strategy will take a few minutes to figure out and then there is no reason to continue with this game.
Upgrades is one thing, but the possibilites are limitless. Would be awesome if you came up with something new. The graphic and feeling are great. The only thing missing is the accual game.