This got me thinking about difficulty selection. Right now, difficulty displayed on the maps doesn't seem to mean anything. Zombie Massacre should have a different option for difficulty. I've listed some of my ideas for difficulties.
I feel that the current difficulty would be easy mode. There aren't many special zombies and you only need to walk backwards and hold LMB. The only difference from the current version of Zombie Massacre is the price for upgrades. Upgrades in easy mode will be cheaper. Its day time so zombies are much easier to spot. You will earn less units on easy mode.
Normal mode will with a mini-boss halfway through. There are more types of zombies and upgrades are more expensive. Weapon drops are less common and zombies are tougher in general. Its nighttime with few light sources like fires and streetlights. Normal units payout.
Hard will have a mini-boss at the end of every day. More specials spawn with upgraded abilities. Weapon drops will be rare and weapons will have less ammo. Upgrade caps are lowered and upgrades are more expensive. It would be nighttime with very few light sources The final night could have two bosses spawn, and/or a shorter time limit. Higher Units payout.
Game difficulty and game length are separate. Shorter games will have more intense waves with increased upgrade points, and longer games will have less intense and less rewarding waves. An endless mode could be added with bosses sent every seven days. The upgrade system in an endless mode would have a sharper cost increase and could have a higher or no upgrade cap.
Special zombie abilities will change with the difficulty
Boomer Zombie: A fat zombie that explodes on death. In normal mode, Boomers will be slower and deal friendly fire damage to other zombies. Hard mode could give Boomers a charge ability where the Boomer rushes towards the nearest player and explodes. Boomers will not damage other zombies in hard mode.
Spitter Zombie: Spitters will attack players from a distance. In hard mode, Spitters could cause a debuffs such as blurred vision, reduced firerate, reduced movement speed, and cause damage over time.
Hunter Zombie: Hunters will either hide in hard to spot places or have a camouflage ability. Hunters will jump at the player once in range. In hard mode, hunters could grab onto the player, stopping them from moving.
Swarmer Zombie: Swarmers will move fast but have little health. They will attack in groups, and come from all sides. This will prevent the player from simply walking backwards and leading the horde. Hard mode could have swarmers move in circles around the player to dodge attacks.
'Hydra': This probably wouldn't be a zombie. Maybe some kind of slime. This enemy will split into smaller and weaker version of its self when shot. If not dealt with quickly, these smaller versions could regroup back into the larger version. Hard mode could give Hydras another zombie's special ability.
My main problem with the current state of the game is that you just walk backwards and shoot at the horde. My idea of Swarmers was to cut off paths in the map and force the player to confront the zombies, although this could also be done with better path finding or map layouts and zombie spawns designed to surround the player with zombies so the player cannot walk around the map, leading the horde.