I'm sure you're going to add a lot but here are some ideas

Upgrades: always my favorite part about shooters. Maybe you don't start with so many bullets coming out of your ship and it increases with the amount of points you get (or things you collect, etc).

More enemies: I'm sure you've thought about this and are making plans but many more enemies of different types and behaviors would be cool (like different colored tanks that move in different patterns or fire different types of weapons.

Moving enemies: running around avoiding fire, roaming in circles, etc.

Enemies that fire upon you: I ran the program a couple times and it seemed the tanks didn't really hit me but that's probably something you're working on (you said it was 10% done).

Cool hidden stuff: Also my favorite part of games. Some little hidden things that reward players for looking around or playing the game more than once are really neat.

Rewards for a certain amount of kills without misses or some such.

Anyway there's lots of cool things you could add and are probably already planning.