I'd like to see a plugin that adds that thin snow block whenever the temperature of the biome is under a threshold after some time. Originally I thought that it should check if a block had snow on the same X and Z it is, but temperature might be simpler. And, of course, it has to be only the highest blocks on a coordinate.
I think I'll implement this. It's WIP, but you can test it. Download the jar, put in your plugins folder and type /snow. The Chunk where you stand will be enclosed by snow It will be extended that snow-biomes will have snow-fall functionality automatically, but this is already a proof-of-concept.
An interesting concept - I've tried your plugin - very nice, until seasons become properly available, this does a nice job at turning an ordinary area into a snow covered area - any chance you could make it change water to ice if there's water in the nearby chunk - would add a nice effect to those wanting to make a theme village during winter time !