suggestion!
I've played this game for a while and have seen the similarities to another indie game called "Clone Drone in the Danger Zone" which has not only multiplayer like this game but also a campaign where you fight multiple low-level enemies, my suggestion is to add a mode where you fight easily beatable enemies that progressively get harder the more you play. again, it's just a suggestion if you feel it would not fit well into the game, then you don't have to make it.