add other mobs (zombie dogs, zombie spiders, after each wave of 10-20 zombies to have a boss with different color) ex. 10-20 zombie dogs green, and the boss orange, then after 5 waves add another zombie dog boss (red color) new maps turrets after a certain level a drone above the character than can help shoot adrenaline pill dark mode (atmospheric environment rain/sun/snow) power ups and bonuses character upgrade (health, speed, weapon skills) customizable weapons tap to shoot instead of the right bottom button storyline based of levels instead of unlimited levels, players can choose between unlimited wave and storyline, where it shows the beginning for each character and where it reaches to an end. Maybe meet each other at final destination and have the same boss? maybe include all characters to fight same boss? ex. same enemy to unlimited waves, also add some explosive barrels, turrets and some walls that can protect you from zombies. cheers, neo.
