Everything is good on paper.

One big flaw is making everything seam together nicely. In a zombie apocalypse, not much is going to be occupied, leaving large, open areas that will cause a huge choke on even high standard PCs. Another flaw is the multiple loading points, having 10 for a single map is a bit of overkill, isn't it? Most players would get lost and go in circles for a while before rage quitting. Really, the only way to stop this is to cut off old parts of the campaign, but that would really kick the idea of a "free-roam city" right in the giblets as you'd expect to be able to back track to collect supplies that you may have missed. That brings me to another point, no campaign is meant to have players re-playing the levels, so supplies would be randomly generated because the maps would reset after the survivors leave that map.

It's not impossible, but it's pushing the boundaries of the engine. L4D was not made for this kind of thing, large maps will most likely crash systems and screw over computers. Think about optimising it a bit before trying anything that is any different/larger than what is currently made.

Hmmmm nice idea and I would help, once i finished my Portal Mod. I thought about something like this too, but it would not be as much fun as you think. Have you ever played Portal or Half Life 2? If yes, you know how annoying it is to wait for the next map to be loaded, while in combat. Imagine this in online mode with 4 players. Suddenly appering and dissappearing people, because you won't enter the same area (that loads the next map) the same time and maybe other computers load them faster slower. I really like the idea, but the Source Engine wasn't made for big open areas like in GTA IV or Crysis. Once I'm back I show you how I planned my campaign, to make it feel like yours, even if it isn't! ...

That's another problem. I have thought about a linear campaign wich let's you decide much thinks so that it doesn't feel linear. An example: Linear: You have to get to point X. You can decide wich door you open and through wich you go through. BUT: You're still in the same building.

My Idea: The same situation, BUT before even entering the building, you acn decide, if you go through it, over the street or somewhere else. You can change the way you're going every time, because the building is mapped 100% and every room has it's things that make it worth going through it. It would be great, because you can choose different way's everytime. And if you make multiple savehouses, you can make multiple ways that lead into multiple maps.