Survivors, welded or nailed door baracades
perhaps part of the reason zombies win so often is because they can freely turn door handles, passing through unblocked doors unchallenged. my idea is either being able to use boards to barricade doors, or being able to pick up a welder to weld fence doors, or metal doors. then, the door has "health" (or strength) which the zombies must hit to break through.
It would probably be a good idea to make zombies able to break easily through weak barricades to avoid exploitation of this addition.