After watching Terraformers at work, I've come up with a list of (hopfully) minor alterations to how they act on the world
Fighting with Minecraft itself:
Chilling: Snow cover blocks should not be spawned ontop of ice as Minecraft removes them a few seconds later.
Irrigation / Cultivation: Turning dirt blocks to grass should only happen if the block above is air. (There are other blocks that allow grass to grow but it'll probably be simpler to let Minecraft deal with grass growing on those).
Desertification: This program spents a lot of time/EU fighting never-ending waterpools. I'm not sure exactly how to remedy this, perhaps it could replace water with sand/gravel?
Possible missing behavior:
Flatification: Appears to ignore snow cover and snow blocks.
Irrigation: This could also remove snow, enabling it to transform icy tundras into lush green areas in a similar manner to how it effects desert areas.