Things that are needed
1) A radar. It gets kind of boring trying to hunt down the last enemy.
2) An option to hold down the mouse button to fire. It could make a player fire slower than say if they were just clicking it.
3) Stronger enemies
4) More types of enemies
5) Mouse pointer needs to be different. This will help get players into the game more rather than looking at the same pointer they use everyday on the computer.
6) Music and sound effects.
Three more thing before I finish up.
The enemies can be hit multiple times when they are dead, which in turn increase the amount of pixels a player can get by a lot.
Also, the pixel counter needs to display number correctly. Once it gets to 1000 most of the numbers get cut off. And, When you go into the shop it says money and not pixels.
Finally, the Health upgrade in the shop gives way to much hp ( could be because I keep hitting dead enemies though, but I had around 400 hp in total.) It should stay at just an increase of 5 at least.
There is still quite a bit you have to do, but these things will help make the game better.