I loved Prison of Elders but the progression isn't as enticing now since the drop of TTK. So I am suggesting a way to have PvE on PvP maps where enemies spawn out everywhere randomly. The levels will get harder and are infinite, possibly, but the rewards will get better. You could have a mix of Taken and Fallen on one level and the next just be Hive. This would keep the players on there toes at all times.