if people have to leave, they'll leave, no matter the penalty. imo its a bit silly to join a dungeon and leave after lvl up (in the sense they don't hit max lvl), one could gain more exp for the next lvl if he was to continue, but if they choose to leave there is a penalty in place for this action. even your proposal is open for exploitation, those people who leave because they lvl up, would stay and afk until you kick them, this wastes even more time because you guys wait and wonder if they'll ever come back. now at least you can choose to fill up this spot or vote abandon and try again if you fear a long waiting time to get the spot filled up.