I could see this as either a challenge mode addition or an 8 player arena mode. A player would start a lobby for others to join. Each player would bring a single character that would be limited by not being a double in the party then they launch the mission or arena. Prizes would be given out as dice to roll against a prize list like in challenge mode. You get one die for beating each boss like in challenge mode or if in arena one die for each of the first three members of the opposing team defeated and an additional random chest for a win. As arena this would add a coin or gem cost to enter 'cause it pays out prizes but could be turned into some kinda future tournament.