A good zombie game would have the following elements:
- Turn based: I'd really love this perfect game to be a turn based tactical RPG. Intelligence (and firearms) give humanity an edge over the living dead; it would be great to have a moment to make decisions.
- Teamwork: You should be able to recruit or rescue survivors with different attributes who bring things to the team. It would be okay if you didn't have direct control over your teammates; perhaps they would have a "loyalty" stat that affected how likely they were to follow your orders. That said, the game should be playable by a lone character.
- Gore: Yeah. We need jets of blood, spattering brain matter, and dismemberment. It comes with the genre.
- Replayability: The game should focus on small human elements, perhaps with goals created along with your character. "Find a secure shelter," "go down in a blaze of glory," "rescue survivors," "get laid," "burn 50% of the city down," "find a place to deliver my girlfriend's child" could all be goals your character might be created to achieve. Each campaign should take roughly the same amount of time to achieve, and each campaign should be randomly generated around your character.
- No overarcing plot: Zombies, doomsday, I think we're good. Left 4 Dead never explains the origin of its apocalypse, and I think it's doing pretty well.
- Online play, DLC, and achievements: Yeah, I do want to be the guy with the "killed 40,000 zombies in a nuclear explosion" badge. I want to play a skirmish with my friends, acting in concert as the US Army's First Anti-Undead Corps, AKA the Big Dead One, as we shoot our way across town. And I want new goals, new weapons, new skins, and new obstacles available for download from time to time. Possibly "bought" with achievements.
- Variety of tones and genres: In other words, the game should be able to accurately recreate the mood of each individual Evil Dead franchise movie throughout multiple plays. One character swims in ammo and cracks one liners. Another hallucinates after shooting his infected girlfriend. Yet another is one of a group of scared people trying to cope with a situation beyond their heads.