good idea , maybe add some different enemies , bosses(big ships with much health?) , mini turrets , achievements(players always like some achievements) , design your own turret? , rain?(slows speed of airplanes) , solar-recharge? (temporary increasing fire rate) ,
my dairy day 7 today i destroyed police men , cars and helicopters oh and i saw some satelites 1 day i hit them for now i kill people now am i upgrading my stats end day 7