I've played through almost all of the Ghost Survivors, and they are some pretty good additions to the game. Like 4th Survivor the goal in 3 of them is to go from one end to the other end of the map with an added unique enemy type per character; Kendo deals with exploding poison gas zombies and flamethrower pack ones, Katherine has Pale Heads(?) which are extra tanky (endurance) zombies, and Ghost gets the shaft and only gets armored zombies which aren't very unique.
After you complete the announced Ghost Survivors you unlock a fourth scenario No Way Out and you play as the Sheriff from the Gas Station trying to survive an onslaught like the Cabin part in RE4. Each unique zombie shows up. You're in the main store part and you start with Adas handgun with infinite ammo and accumulate more guns from backpack zombies that will come in. You have to kill 100 zombies to finish. Haven't beat this one yet but I'm really glad they included it as well for something less linear.