I see having 2 options in regard to a 'Survival/Unlimited' mode;
4-8 players on one the existing maps each on their own. This would be a solo PvPvE mode. Choose the best spots on the map to survive the hordes, battle to keep your spot against other players or form make-shift alliances to defeat the other players. Victory could be determined by total time alive & number of zombies killed perhaps? I would imagine this is a mode with re-spawns.
4 player PvE co-op mode in a smaller designated area with available defensive upgrades and equipment replenishment. All players working together to hold a specific objective for as long as they can. This would be an unlimited time mode that would progressively get more difficult the longer you survived. Motivation/Story could maybe be trying to hold a strategic location to let survivors flee or holding the horde in place to allow a air strike on the city?
These are both off the top of my head and clearly need plenty of specifics and details ironed out but would love both of these modes.