Time to think of about the theme

So before I start to develop the game any further I want to figure out what is the theme for my game. When I have a clear theme set up as early as possible it will be easier to come up with new ideas that improve the game.

I first though that this could be some sort of artillery command game. However while playing around with my first prototype I noticed that I want to make objects to move towards bottom with different speeds. This will make the objects overlap when the faster object will pass the slower object. For me it feels very unnatural when land objects (soldiers, tanks, etc.) overlap this way, so I had to rethink the theme a little bit. Then I remembered that in most top-down shooters you are fighting against multiple enemy planes, which may travel across the display with different speeds. In these kind of games it does not feels so unnatural when the objects overlap. So I had the idea that maybe the theme could be an anti-air defense game.

So the game theme will be World War II themed air defense game. In this theme the story could be something like this; The Axis planes are attacking the Allied factories and your anti-air artillery is the only thing between them. If the enemy plane is able to get past the screen the player will lose health, which indicates the factory damage. If health reduces to zero the game ends as all the factories are destroyed and enemy has reached its goal.