EnderMist

Endermist would be something, that could get out of hand in the players world if they aren't careful or are too careless. this mist would be almost like an event or rather like an opening from the end, perhaps some structure pervades this mist. This mist would make all undead mobs more stronger, even them having new forms or eyes. another thing is that, the player although overcome with this challenge, would have a reason to also let this mist grow or even have a reason to go after these undead mobs, as they could have unique drops or other such properties, perhaps even the endermist themselves could be of use. also one other thing of note, from another post, gold could be somethign that really is the only thing or has more of a chance of doing significant damage or having a chance of doing more better damage against these mobs. Thus gold would garner more of another use, and also force players to bring along lower tier gear instead of just discarding them.