The real time disease spread sounds cool, though. I wouldn't mind being late to finishing a quest and find that area completely infested. It would make quests harder to finish or change up quest objections from "rescue this person" to "kill this person" if they got infected.
Yeah, your right. But what if they could come up with a more forbidable oponnent rather than a slow moving, mindless, non-agile, non-challenging opponent? Oh wait, that IS what GOOD game developers do. Throwing in traditional zombies cheapens the gameplay experience because there isn't much to fighting traditional zombies. You won't need to time your attacks or dodges like in a skillful game (like Dark Souls) because zombies are slow, have short reach, and move in predictable patterns (straight at you). So the only way to remedy that is to change zombies to act in a different way...but what would be the point of using zombies, then?
Not exactly, you forget the game itself must accommodate to making the monsters in the game more challenging. For example, claustrophobic set pieces, sparse ammo, clever enemy layout or even adding cool concepts which doesn't effect the zombie lore too much (The Crimson Head concept in Resident Evil Remake was brilliant). It's a joint effort in environment and enemy design.