Been running a SMP server with some friends for the last week or so. Been exploring various mods (using hey0's currently) and occasionally editing the level to create unique features for players to explore. The level started as a snow level, but the general concensus was that it was annoying so it got shut off, and all the existing snow and ice removed. One section of the map was left snowy though. A few days later I pushed out to generate new chunks and then mass replaced everything to generate a desert. However I think it could be better if it wasn't very square.
Proposal: Create a server mod with a slash command /makedesert [#] and /makearctic [#]. The mod would then mark the player position as the center of the new environment and begin converting blocks. The mod would begin from the marked position and convert every block within # radius as appropriate (with a depth limiter). For a desert, I converted all water to air; dirt, grass, and gravel to sand; leaves to air; trees to cactus.
This doesn't need to be a very quick conversion, a few dozen at a time would both help avoid undue CPU load and create the effect of the desert "spreading". Doing it this way also allows the creation of a desert without having to explore and generate all the chunks and then convert them in an editor, which can involve dozens or hundreds of millions of blocks.
I'm unsure what to do at the borders of the environment. Having trees and grass just suddenly start looks awkward. Not to mention having the border circumscribe a body of water leaves you with a wall of water hanging in air. Currently I've just inserted a wall of sand up to the water level, but it looks unnatural.