In swamps, there will occasionally be a structure called a Moon Tomb. It consists of a small above-ground stone tomb. Normally, this tomb is not active - it is essentially a decoration. However, every new moon it will become active. Small mobs called wykes will spawn in the area, carrying candles (and providing dim light). They will defend the area by blowing poisonous darts at any not-undead mobs (e.g. animals, spiders, villagers) or the player, and when killed they have a chance to drop these new candles - an interesting lighting and decoration block, which if a change to the lighting system ever happens could be yellowish in colour - or blowpipes - a quick-firing but less damaging equivalent to a bow. 

If the player reaches the tomb, and along with a bright flash effect of some sort, they will find positive (e.g. healing and regeneration) potions along with an item with a name like 'Moon Fragment' that can be placed somewhere to decrease (or remove) mob spawn in an area, and also provide a bright light over a large area, or maybe night vision instead. There will only be one each tomb, to prevent the player from being able to place them everywhere. This way, mobs aren't invalidated.

So that's a grouping of ideas into one thing that players can explore to look for, since it provides both some new items and a useful defence or 'power-up' thing