Highlands aims to be a comprehensive improvement of Minecraft biomes, trees, and terrain. If you've ever felt bored exploring the world of Minecraft, this is your mod. In 1.8, Highlands is going into uncharted territory, where no biome mod has gone before. It's been designed from the ground up, and features from the old 1.6.4 Highlands will be added in as they are updated. Highlands also gives you an incentive to explore into uncharted territory, whether to search for the perfect mountain to build a base on, to complete your botanical garden, or to search for more lucrative mining opportunities.

Features:

18 new biomes!
8 trees, all with their own wood, leaves and planks!
6 small plants!
Custom world type and mountain generation!
Fully compatible with customized world settings!
Settings to disable biomes you don't want!
Biome rarity fix!
Ore generation changes per biome!
Forest generation based on height!
Improvements to vanilla biomes!

More realistic generations, biomes are not a thing anymore (instead an area is characterized by temperature, humidity, and altitude, allowing for nearly infinite amount of "biomes").

Trees no longer are boringly structured, they are generated realistically, using a geometric principle real trees are using - allowing for dynamic shapes that vary based on the area they grow in.

Caves are much more interesting, less "tacked on" feeling. The generation of the world will also not be flat, there will be less oceans and in general it will be more like real life in the way things look (with all of the beautiful structures nature creates).

Start by creating a new world. Then click "More World Options". Then click "World Type" until you see "Biomes O Plenty".

The zombies should be able to spawn in any biome derived from the BiomeGenBase class. Zombies should spawn in biomes created by ExtraBiomesXL or Biomes O Plenty for instance.

I recommend creating a world with these presets if you want a real zombie apocalypse adventure.
2;7,56x1,5x3,2;6;biome_1,decoration,lake, lava_lake
This is a swamp world similar to Swampcore, only deeper. You can mine ore here. Using the CrackedZombie defaults you will have to contend with many walkers and spiders. Want some bad weather to go along?
/weather thunder 100000
should do it. Oh, and watch out for the witch. With a little luck you can find a witches hut for shelter on your first night.