Spriteworx

Add a de-bounce feature for the start screen menu selections to prevent menu cycling when the button or space bar is held down.
Allow the "Enter" key to be used for menu selection (in addition to the space bar).
Allow directions to be mapped to the cursor keys (if possible).
Allow cursor keys to be used for menu selections or put instructions on screen to use "Q", "A" and spacebar.
Allow exit from the game while it is paused.
Add some sort of cool overall evaluation at the end of the game (perhaps a rank?) to easily see how well you did. For example, "Congratulations, you reached level 33 with an 86% kill ratio and achieved a rank of Super Battle Commander!".
Create a map editor and have an online portal for users to upload their custom maps for others to download and play.
Have a 2 player mode where one player secretly selects the type and timing of the enemy units that will invade the map for each wave, then the second player is the one placing the turrets. Player one receives points for un-killed units.
Add in-game instructions.
Have an attribute box pop up when you hover over a turret to tell you what type of vehicles it is effective against.
Make the range ring for the turrets more visible.
Add more detailed graphics for the buildings.
Add rumbling sounds for the tanks.
Have different sounds for the bullet impacts depending what type of vehicle they hit.
Have a way to pause the game and wait for you to finish placing turrets between levels so you're not rushed? Still should be able to place more turrets during the gameplay though.
Have different graphics for different turrets other than just color change.
Have different sound effects for the different types of turrets.
Have the slowdown shot make the vehicle slow down permanently?
Put a counter on the screen indicating number of turrets remaining available to buy.