Adding some points onto this
3: I'd like to see what someone else mentioned as upgrades or alternatives to the pistol, the pistol itself feels lacking in many ways as it's slow, does little damage, and just isn't all to interesting in and of itself. Maybe something like dual pistols, an smg, maybe even a magnum that effectively one shots most but has a painfully slow speed.
4: Blimp mode for the player, this would be an unlockable mode and doesn't even do much, other than maybe increasing how much hp the player has, maybe like an extra 50 points, and when the player does go down it draws out the ending inflation where they get up to the blimp enemy size before ending with an all mighty kaboom. Also maybe the ability to zoom the camera in and out during game overs, have had the camera either get caught on something, or the player gets pushed to close to the camera and most things just end up getting obscured by a singular tit or belly.
5: A reload speed upgrade wouldn't be remised I feel for things like the shotgun and sniper as they feel almost useless past a point in the waves because they reload one shot at a time, and the sniper is painfully slow with this as well.