I was thinking maybe some trap door spiders put around the map which could also be a place you could find items and things the spider has taken like ant eggs and other things and when you leave after killing it and looting it's home the door locks and after some time it respawns and opens back.... a somewhat semi- boss if you have multiple ones and multiple respawns or just make it something like the broodmother
Also since it will be underground maybe being able to make some kind of glow sticks you can throw to light up dark areas would be a good idea
