You add a vote system, so you can get to call a vote if you wanna give up the dungeon or not, if it is still timed, 4 out of 5 players vote yes… .you can leave without penalty, if less than 4 vote yes, you get a penalty, furthermore, when your key is out of time 2 out of 5 votes are required to leave a key without penalty. To add to this, once the team has 20 or 25 + deaths, you can leave without penalty, to avoid hostage taking, at least 50% of the deaths MUST be from other players, if the deaths are from the same player, it doesent work, to avoid sabotage or something like that. I dont know what punishment would be good, like make the player not be able to use group finder or any1 with him in group to use group finder, they can do mythic + with their guild/friends, but not over dungeon finder. Or, make them lose 1 weekly reward from m+ and have to farm it up again, or dunno, make it if you leave once you get 1 day no group finder, leave twice in a short period of time, 3 days no group finder + remove 1 reward and he has to do 5 more mythics to get it. And yes, the key isnt saved, its depleted, but the player is punished for sabotaging.