I think you you got most of what I'm saying. Since there are achievements, it would be cool if the story was so open that it is possible to get acheivements based on who survives. Say there is a character that is a 100 year old woman. She's slow, on top of that if she get's too scared or you take too long; she dies. Escorting her through the game would be an intense challange. Also, I think it would be prudent to have infinite zombies because we are assuming that a whole city is after you, the zombies should trickle in through specific areas like broken glass door. I don't think you should be confined to the mall, but leaving it would be foolish because you would get mauled in minutes. Then again, jumping out the front door, hotwiring a car, and leaving town, could be an achievement.