A world type with some flatter biomes

Many java mods for Minecraft adds buildings and stuff, like Millenaire. They need areas that are fairly flat and have fewer ravines etc. All these mountains and ravines etc. are nice to look at and explore but they are not fun for these mods. For me the word "plains" means something large almost flat. And that cannot be said of the current implementation.

So I suggest a new world type where applicable biomes have some areas that are much flatter then what they currently are. I.e. not super flat, just flatter.