I don't have that much theories since I play zombie mode a few times.
However, what I noticed are these things :
- The idea of a "reinforced bus" (the final scene you can see a zombie tearing off a woodden barricade while the bus rides) riding on zombies seems to come directly from the final scene of Zack Snyder's "Dawn of the Dead 2004", where the group of survivors uses reinforced vans to escape from the besieged shopping mall.
Now that is some good reference ! :-)
- Also, the fact I saw that zombie tearing off the barricade leads me to think that there will be a part of the game where you will be in the bus while "something or someone" drives it and you will have to counter zombie attacks while moving on the road.
My theory ?
I don't remember WHO said it before me, but I agree with his idea : I guess the developers have made bigger maps like they promised us and that the bus will serve the player to travel from one part of the map to another.
After all, the developpers announced a "zombie mode as you've never seen it before" and said it would be "much bigger than the precedent ones".
Probably the map is divided into "sections" or "levels" (I am no game developer, so I don't know exactly how to call it).