I would love a few more levels with increased difficulty.
And you could add a "high score" system - how many clicks on a level player needed to pass the level. You could then separate players that think from players that only click randomly
Could use a few bugfixes... The AI is stupid, my both soldiers were trying to shot an alien that was behind a tower, and they were only shooting at the tower, the tower itself (twin tower) couldnt hit the alien, despite that the alien was just in front of it. The green shots from the aliens explode too far from my soldier and still take som hitpoints.
I miss a save game function. I didnt want to continue after 6th level, so i just let the ship stand at the bottom of the screen, and to my suprise, i passed the last 4 levels sooner than i could get killed... I moved the ship only in the fights with bosses. Sometimes not even there