I think an ammo system for the weapons would've worked better, where you have the weapons in some kind of inventory and can switch out between them. Or maybe be able to only hold 3 weapons in your inventory or something, and the aliens would drop ammo. That way you would be more motivated to unlock all the guns and such. After the first 6 levels i had all the mid-range weapons and i wouldn't want my aliens dropping short range or long range weapons so i decided to not branch out. Two levels later; i have 20 something crystals in stock, and nothing i can do with them. It would be awesome if the game got you more motivated to buy all the weapons.
Besides that i think the graphics are great, the story is really amusing, the adding of new monsters keeps the gamplay fresh, maybe some more space to move around would be cool? Instead of just another backdrop.
Great game, i think the developers have some great potential.