tl;dr - District 12 (the parallel world) is now open! It has mesas! See below for portal instructions
There are a few missing biomes in the overworld (notably mesas) thanks to some weird circumstances surrounding its creation. We've toyed with a few ideas on how to solve, including expanding the world border (there are no mesas whatsoever even with a 12k block radius world), artificially creating one within the world (ugly), expanding the world with another generator (extremely ugly), and creating another world with a new generator. The later was chosen, and that world is District 12!
District 12 is a parallel world to the Overworld, with a 1-to-1 coordinate mapping (unlike the nether which is a 1-to-8 mapping). This means that if you are at X-42 Z-42 and enter a portal to District 12, you will emerge at X-42 Z-42.
This world operates exactly like the overworld, though one word of caution: the mesas will get plundered for resources. Best not to build anything you want to stay around permanently there.
So, what are these portals of which we have referred? I'm glad you asked! You will need regular obsidian and a lighter as with normal portals, plus a coal block. This coal block is crafted from nine coal, like so:
Once you have those coal blocks (you will need two), place them at the bottom corners of your portal, like so:
This works with portals of any size:
Congrats, you have created a District 12 portal! They only work in the Overworld, and portals created inside District 12 will lead back to the Overworld (not the Nether).
Why the Name?
District 12 == Hunger Games == Coal Mining == Resource World