Proposed mechanics:

• Rather than mindless pressing forward, a mob could retreat when wounded.
• Heroes prioritize mobs which are still pressing the attack
• Enemies which retreat 2 rooms from a lit room or hero vanish, and join the next mob wave
• As applied to all mobs, the mechanic would require much more hero-intensive play
• Alternatively, the property can be added to a few specific mobs.
  
  

The result: waves balloon more dramatically as a dungeon level progresses