Thank you! I'm glad that my little game was able to provide fun even though it's still so bare bones. :D Sorry there is so little to it at the moment. I'm working on it!
I've already fixed the ceiling issue so that will be in the next update. About the bat idea - I actually have something very similar planned! ...But it's coming in version 0.0.4. Not exactly bats but little quadrotor drones that zip around. The new enemy in the upcoming V0.0.3 update is a tough security robot, and I've added an SMG with a high rate of fire and physical reload ability to help fight it, though it's not necessarily a straight upgrade to the pistol.
Also, about adding health; I have finished adding health restoring items (food that you eat) that replenish HP for V0.0.3, and difficulty options are probably coming for V0.0.4. Also, now you can see how much HP you have remaining when you look down.
I've still got a few more features to add, then I'll get some more bugs out of the way and the new update will be done!