I like this! Reminds me of a (space) game I loved to play that had a bit of role-play story, and at the same time was free to explore, shooting ships and unknown aliens that would appear in peculiar places, but would not repeat (had a level of randomness).

The best of it was the story embedded, which had clues for you to find upgrades and things like that. Slighly different from this, but that's what triggered me to remember.

Suggestion... Obstacles would be interesting in this mode, but not just static or dynamic ones. That would appear and disappear kind of thing.

Also, about firing modes... One of the things that was epic from the old times was buffered attack modes. Like long-press the button until it charges to the max, and then let go to launch the big cameeeameeee! LOL

Will challenging others be possible? People like to compete on these things.