Once this mission is complete new side missions open up to save civilians. (gameindustry.com)
Additionally, new side missions called Distress Calls have been added that have Frank rescuing survivors and then leading them to safety while getting away from a group of zombies. (playstationlifestyle.net)
Still a lot of different side quests / missions to keep you occupied within the city when you are not doing the campaign or just want a break from it like for example if you take out what is called the D.U.P Mobile Command spread across the city you will get new side missions, blast shards show up on the map and you get new respawn points. (thegamersparadise.com)