Nether Colonies. This is a futile and odd idea, but it may work well as a very rare ocurance in plain and desert biomes. Half as rare as diamonds, probably.

Story:

This world and the nether were deemed to be seperate a long time ago. That was before the colonies. The undead swine have arrived through unknown means to the normal world, populating with these strange colonies. edgeless 5x5 homes with iron bar doors, soul sand pig pens, giant tanks of lava, and glowstone lamps. Seems like these creatures have made a decent home here. Lets just hope they don't get completely comfortable. We already got one nether.