I am left in awe of this game
This game is absolutely amazing in terms of entertainment value. As it is I have no complaints or problems with the how the game plays, very very well done. But...
If you decide to do a second game (which I think many people besides myself would love to see) I would suggest a couple things...
A mode where you didn't have a base to protect and instead the zombies came from all around perhaps would be interesting. It could be done in ways with upgrades between or perhaps like survival mode in this game, or both even.
A weapon that "draws" the zombies to you for base mode perhaps so you can draw them off your base in a pinch
If you keep the laser, I'm going to mimic earlier comments, and suggest it would be more useful either one with more shots before reloading, or two a lower reload time
A second game would be awesome with more weapons available to use, though that might require a new way to scroll through them.
And finally perhaps if you wanted to adding in "achievements" could be interesting, though the concept is getting a little overdone.
Still, this game is amazing and a follow up would just be icing on the cake. Fun game, definitly worth killing an hour or two on. I would HIGHLY recommend taking at least one try at each mode. GREAT JOB!