Bukkit

This plugin is already pretty great. What I think would be awesome would be changing biomes. When a player cuts down a tree on the edge of a forest, it checks the chunk and if there are no trees left then it converts it to a desert, plains, or tundra depending on what biome is bordering the forest. If it converts to a desert then the grass would slowly become sand and cacti would spread into the area. Then when a sapling grows the plugin checks how many trees of that kind are in the chunk. If there are enough trees in the chunk then the chunk is converted into that tree's biome. Any sand in the area that isn't near water slowly becomes grass, any snow in the area begins to melt if the chunk became a forest or jungle, and if the chunk is now jungle some bushes and vines might spawn.

This would give users an incentive to prevent deforestation, because once the deserts have spread far enough it would become quite difficult to reforest them. It would also mean that areas overrun by spruce trees would get snowfall, and that when jungle saplings spread enough the area actually spawns ocelots and resembles an actual jungle.

Another (much smaller and easier) enhancement would be to make oak saplings in the swamp grow into swamp trees and any saplings that spread from a swamp tree to also grow into swamp trees, since swamp trees are shaped differently than regular oaks.

This would be pretty amazing :D