I have found that two realistic biomes that can be made with the terrainmod (I cannot get a link, the Minecraft forums are down) are extremely useful in very different ways.
Rainforest. (Modified from original, very dense trees, no open areas.) Can be used to produce vast amounts of wood, and give you a safe haven up in the canopy, as mobs do not spawn on trees.
Northern mountains. (Not a normal biome, made with low volatility settings for smooth tall mountains, very little trees and tons of iron.) Can be used to make a very successful mine, a wonderful tool production line, and the large open areas of flat land offer a snow covered landscape to build giant castles on.
The two are vastly different, and if placed miles apart on a multiplayer server could offer realistic trades of resources. Wood being exported to the barren mountains, iron axes and tons of ore sent back. Adding in a gold filled barren desert, a diamond filled African landscape and a thick swampland could add even more diversity. If each biome was thousands of miles from another, stretching further than a real life day's travel, connected by train stations in the nether powered by thousands of boosters, you would have a completely epic server. Until it is griefed.
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