"Enhanced" world-gen

[xoft]

Some of what you want may be already doable, just playing with the generator settings. Some could be done with a plugin. Unfortunately, most of it is very difficult, if not impossible, to do. You'll notice that these kinds of pictures are just that - pictures of something that someone spent an awful lot of time designing and editing just for a screenshot. The terrain is custom-made for the original situation, or built from scratch, but always with the bigger picture in mind. However, terrain generator cannot work like that, it cannot take too much of a "big picture", because it is very costly, in terms of performance, to do so. The tree generator requires 3x3 chunks worth of terrain composition and 5x5 chunks of biomes around the currently generated chunk, and that's already pushing the generator to its limits. Now imagine you wanted to do a terrain feature that spans several chunks and needs to smoothly transition into possibly tens of chunks in each direction. That is simply infeasible.

As for the plugins, don't go there yet. The OnChunkGenerating and OnChunkGenerated hooks are good for general notification, but not much for tweaking the generator. If you tweak too much, things start breaking, such as villages mid-air or trees in the Sahara.
I have a plan for a better system of generator API, more tightly coupled with the ComposableGenerator pipeline, that would allow plugins to really tweak anything and everything. As usual, reality can hardly keep up with the plans