What about a dlc where your home base is a train that stops at randomly generated towns?
Each town could vary from plague infested to basic zombies.
You would have to find fuel to maintain the train and sometimes spend materials to repair broken tracks all while upgrading your home base on board.
Since it would be smallish maps the train would stop at make no vehicles available and up horde spawn rate for a challenge.
(Please add ideas if you have any)