How bout After-Math mode. In which it takes place after the story. In this one there is much more zombies and alot less ammo. Supplies are harder to find, and so is weapons.
And Escape mode. This takes place AFTER After-Math mode, in which you have no weapons at all and no rations. All you have to do is run to the end of a selected area, In which City Center is the end. How bout this for some courses, if you like the idea.
Street- Quickest run through, not to many zombies.
House- Easiest run through, no zombies.
Docks- Medium run through, medium ammount of zombies.
Culv-a-sac(It's wrong, i know)- Medium run through, medium ammount of zombies.
Neighbor Hood- Long run through, medium ammount of zombies.
Suberb- Long run through, large ammount of zombies.
Outer City- Long run through, low ammount of zombies.
City- Medium runthrough, medium ammount of zombies.
Inner City- Quick run through, large ammount of zombies.
City Center- Very quick run through, very large ammount of zombies.
Edit: Here is a huge mode. Surivor mode (No this is not were you kill zombies) This takes place after Escape. If you could program this there will be 2 types of it.
If you escaped by chopper you will end in the snowy mountains.
If you escaped by boat you will end up on a island.
Mountains- Harder mode of the bunch. In this one you must find a way to survive for 100 days (you can edit that if you want), or you can find a way out (useally the hardest way you can win), You can use your downed Heli-copter as shelter until day 10 (if you use it after day 10 you will start loseing health by little bits, if stay there until day 20 you will die (remember you ca edit this))In the heli you can find the following.
Rations (Any day, search offten)
Knife (Day 1)
Fire Starter (Day 3)
Riffle (Day 7)
After that you are all on your own. If you get the riffle you will only have the starting ammo that you get for it.
After day 10 you will have to find another shelter (a cave?). And so on and so on.
But after day 80 (im guessing the days will be like 12 minutes?) there will be a winter storm, in which if you stay outside for to long you will die. You can find food here, but very rarely, To get food here you'll have to kill an animal, such as a bear.
Island- Easier of the bunch. It's the same way as The Mountains except you can't die from staying out too long, or from your shelter. Your boat can be your shelter for the entire mode. Since this is an island, if you sleep anyware besides your boat (like in the jungle area in the center of the island) you can die by animals. In your boat (which is way to far on land) you can find the following
Rations (During anyday)
Knife (After day 10)
Ammo (any day after 30)
Pistol (After day 30)
Riffle (After day 50)
Sniper Riffle (After day 80)
Flair (After day 90)
Since you can find alot of break able stuff on the boat, the Repair Man would be a pretty obvius choice here. You can find stuff on the beach (such as fish, if you can't find alot of rations). You can find weapons in the jungle area (in the middle of the island). But if you stay in there too long, animals can attack you.
This was very long, toke me a long time to type too. Hope you like it. You can edit whatever you want on this Ash. It's just an idea.