Needs A.I.
Consider your audience. Most people who go to these flash portals are by themselves and won't have easy access to playing your game with multiple people. Therefore, every multiplayer game should have optional (or by default) A.I. opponents. I realize this makes the game harder and take longer to make, but the game really needs it. At the very least you could have had a mode where you face multiple planes that are already in the air flying across the screen and shooting at you, and record how many you shoot down before dying. They don't even have to do a lot of movement.