I really enjoy the arcade aspect of this game, addictive and kind of relaxing. I like the fact that you can go for getting the highest score or just shoot and relax.
Things that I think you can do to make it ''releasable'' are: using the right click to use a different weapon or move in a different way like a teleport, adding power-ups, adding a challenge mode like killing a set amount of enemies in a minute or only killing a specific type of enemy while avoiding others, online leaderboards, a mode with bosses. Hope some of those ideas are useful.