You don't need the mesa biome to get stained clay. All you have to do is put a clay block in the oven and then combine 8 hardened clay with a dye. We won't be seeing new biomes with the upcoming update. The only terrain change coming is the addition of a new structure: the witches hut.
The Meaning of Life, the Universe, and Everything.
They say the only terrain change is Witch Huts, but they also say they added Horse Armor. If you have already explored your world, there will be no horse armor (dungeons, strongholds, etc. have all generated whether you found them or not).