This idea comes from a different game but could be incorporated into this game.
Star Wars Galaxies had a treasure map system that was fun I will explain it here.
First the player would click a button to search the ground.
While searching the ground the player would get No Find, or Weak Mob Spawn, or a Treasure Map about 1/100-1/300 chance.
Player was required to move before having another chance of finding something.
Second the player would open the map and it would give coordinates for the player to travel to.
Third the player would arrive at the coordinates activate the map again and find a treasure box.
Fourth the player would attempt to open the treasure box but get attacked by mobs that would spawn in around the player
The spawns and rewards inside the treasure box would increase based on the size of the team.
Fifth only after the spawns were all dead would the player then be able to open the treasure box and receive the reward.
Of course there was a time limit after the treasure box has spawned in so that it would be removed from the world.