If you want to infect The WORLD, you need not common zombies, like:
1) Thinker zombie. He don’t walk away from the people. He WANT to infect them. He can do chase. Accessories: big head.
2) Spittle zombie. He spitting very long away... Accessories: growth in back, like backpack.
3) CrAzY zOmBiE. iF hE sPiT oN tHe PeOpLe, He DoEs ReD aNd CaN sPiT aGaIn. Accessories: I don’t know 🤷♂️...
4)Sprinter zombie. He’s movement very fast. Accessories: whatever you want for legs.
5) Sniper zombie. He can spit like fireball, and if it shoot the people, it EXPLOSIVES! Accessories: long mouth (like Peashooter from PvZ).
I think you can add laboratory in the game in main menu... Player can have cards on zombies, when he play, and thanks to brains, or other money, player can UPGRADE the zombies. And please, do game music and other songs. Add in the game more paints. And when you zombify first zombie, do arrow or other sings, where first zombie spit.