Some biome ideas, plus tweaks and items for existing biomes:

I have a few ideas for biomes, corresponding items, and mobs.

Salt Flat: A large expanse of salt leftover from an ancient sea. This biome's surface would consist of two new blocks, and one new item. First off, the top of the salt flat will be covered in a thin block of salt similar to snow. Secondly, beneath that, there will be salt blocks, which can be crafted with 9 pieces of salt in a workbench. Using a shovel on a salt block will drop anywhere from 3 to 9 pieces of salt, and using a shovel on the layered salt will drop 1 - 2 pieces of salt. Perhaps salt could be used in brewing in the making of potions of mining fatigue, and if corrupted, potions of haste. Salt could also be used by mods that use it, such as Harvestcraft. Sometimes, hypersaline lakes could spawn, which if used in place of water in brewing potions, can create potions with tier 4 effects.

Quaking Aspen Grove: A large group of aspen trees descended from one tree in a process known as suckering. This biome is inspired by the tree known as Pando. http://en.wikipedia.org/wiki/Pando_(tree)

This biome would include one new tree, aspen trees. Aspen trees have all the same uses as regular trees do. Aspens could also spawn in various other forests.

Shield volcano: A large volcano that resembles a warriors shield. This biome would have a few variants, such as an insular shield volcano, which closely resembles archipelagos such as the Hawaiian archipelago, and the Galapagos Islands. Another variant would be the cold shield volcano, which resembles areas such as Skjaldbreiður in Iceland.

Desert orchard: A desert dotted with various desert flowers, and date palms, which grow dates in a fashion similar to cocoa beans on jungle trees. Date palms can be grown on dirt or sand.

Atoll: A ring shaped biome that spawns in oceans, formed by coral reefs. Limestone replaces stone in this biome.

Barrier island: An island that spawns off the coast of continents. There would be three variants, temperate barrier islands, tropical barrier islands, and cold barrier islands.

Coral Island: An island formed from coral. They can commonly form archipelagos, and may spawn near atolls. Limestone replaces stone in this biome.

Pine scrub: An arid biome dominated by shrubs, oaks, and longleaf pines.

Mediterranean: An arid biome dominated by shrubs. Olive trees can be found growing here, and will grow on trees similar to the apple and persimmon trees.

Rocklands: A subtropical biome characterized by outcroppings of limestone, and a large amount of slash pines. Limestone replaces stone here.

Steppe Tundra: A tundra like biome with little vegetation, plus outcroppings of rock and sand. Plants occur here in small patches, plus the occasional stunted tree.

Tweaks and additions to existing biomes, plus miscellaneous items:

Persimmons should spawn in orchards.

Grasslands and prairies should get their own mob, the buffalo, which, can be bred with hay bales, and drops buffalo meat and leather when killed. Buffalo meat restores 6 units of the hunger bar. They can be bred with mooshrooms to create mooshalos, which can be milked for mooshroom stew, and will drop leather, mushrooms, and buffalo meat. Buffalo can also be bred with cows to make beefalos, which can be milked for milk, and will drop leather and buffalo meat.

Limestone should be added and should sometimes replace stone in biomes. All biomes have a chance of having stone replaced by limestone, but, some biomes always have stone replaced by limestone. Limestone, when mined, will become limestone cobble, and putting it in a furnace will create limestone once more. Limestone can be crafted into bricks, stairs, and slabs. If one puts limestone in a furnace, it will create lime dust.

Coquina should be added as a random spawn to the surfaces of beaches. Coquina is rock made from the shells of small animals. It could be made into bricks, stairs and slabs. Coquina could also be found in oceans. Putting coquina in a furnace creates lime dust.

Roman concrete should be added, and be made from lime dust and ash. Roman concrete has the blast resistance of end stone, and increases the speed of any player walking on it. It can also be made into bricks, stairs and slabs. Roman concrete can also be dyed.

Smoldering grass blocks should spawn in volcano biomes.