Based on the idea of RIOT, I think it is possible to incorporate zombie battles in APB as its future contents, where random people in the streets are replaced by zombies that automatically attack players. The idea is that as the number of players get less, more and more zombies will pop up in the streets. So instead of closing down the zones in the map, players are gradually overwhelmed by zombies. Their goal is to get to a safe spot where they can be evacuated from the city, just like the ending to the RIOT mode.
The only challenge is to program the zombies (previously pedestrians) in the streets to attack player.