This game is really fun and a great way to pass the time. There is literally nothing wrong with the current content, and that is the problem. There is not enough content.
The amount of levels is decent but adding around 20 more would improve the game massively. Also the layer above the spaceship should be replaced with the mothership enhancing the alien feel of the game. And surely there should be some ground resistance in military base and maybe suburbs? Anti-aircraft Guns, tanks, mobile missile stations, the whole lot. More ability’s for example: alien swarm, mothership beam, and heal (if the ground defence damages the ship or mothership). There should also be passive and active ability’s that you buy in shop, not collect in-game. Passive ability could be faster abducting beam and active could be invisibility for ten seconds. Alien ship and alien skins, please! Multiplayer co-op mode? Or multiplayer ground defence vs alien ship?
I know these are a lot to ask for, but hopefully you take some of these these into consideration.
Overall: great potential.