Imagine a WotLK pre expac event except that its Keepers invading cities tossing sphere prisons on people and those slowly make their way to the city exits where if others dont break you out you get turned into a keeper to capture others. Then there would be a Keeper boss every hour that needs to be burned down quicking cause he walks the cities capturing in mass (5-10 at a time). The debuff for the keeper form players would get would be like 45 secs and at the end would drop your health to 1/3 making you easier to catch with a 5 sec slow debuff.
Just off the top of my head.