You know the game TwoGuns, right? What about that game, but with ZOMBIES! Pretty much an infection gamemode. It's the perfect game for it so why not? Blockland has one, TwoGuns should have one too! I'm not picky on how it turns out. So, yeah. A few ideas.
1.Zombie spawns for maps.
2.If a zombie hits you, you become a zombie.
3.Etc.