While I personally wouldn't mind being able to make my Prisoner stronger, I understand why many have reservations about awakening the Prisoner. Currently he is a tactical decision, a compromise between having his powerful passive at the cost of his underwhelming damage abilities. He hits like a wet noodle, or at least mine does. Making him stronger and a more well-rounded character could risk turning him from a situational tool, to an auto-include, especially now awakened characters have a passive as well as a fury.
Grim Reaper would be an excellent choice, due to him being an achievement reward character. Would make it easier for mid-range players to get their hands on an awakenable character.
Looking forward to comparing Saints list and the Official one to try work out which characters are already being worked on :P
The eight hour duration for that place is such a wonderful catch-up mechanic. LoL is good for your first couple of 4*->5* evolutions, but awakening is a whole different kettle of fish.
I've heard Airstrip and Gangland mentioned too right? Haven't done either of them myself yet though.
This would only work if the keys were available more often. As it is currently, it's a nice treat for people whenever the Skull Quest event swings around. Wouldn't be too surprised if further down the line a similar style dungeon with a low difficulty and high duration was released with a focus on the other coloured shards.