Use the stairs to go "out" of the mine. You could actualy start whole map on upper surface and work your way down.
On surface you be getting some progress, but to go down the mines people would need to stack up on money to rush thru hords.
Or differently there would be 3-6 mini boses (small ender man wich you need to kill for ender pearls) while some people hold the cave entrace from on going hordes of zombies.
Would be also neat to have something like red moon or spooky night outside basicly to justify ubnormal zombies spawn ;)
Hell could still be a place where you get THE last pearl