When I first heard of custom blocks I thought they were meant to let the players build their own semi-random structures and put them in the world save together with a few spawning rules so that the world generator could automatically create such builds like it does for villages, temples and so on.

Everything Mojang said or hinted about structure blocks is only meant for map makers, though, and IMO this would be a huge waste of potential: map makers already found a way to generate builds on the fly, in a very inefficient and convoluted way, so if structure blocks were limited to them it would be a nice help, but it would not open a new world of possibilities.

Think about it: people creating all kind of procedural structures and sharing them on websites, so that one can tailor its own SSP experience. People could create a large variety of dungeons, villages with actually different building styles, custom trees, literally anything one could think of, and then anyone could pick the ones they like and put them in their own worlds. Servers could create their own structures and not share them to provide an unique experience. The possibilities are just mind-blowing.

The current system only needs a file for giving rules for any structure: in which biome(s) it's spawned, how many "rooms" it should have and so on.

I know it's not final, so maybe Mojang is already headed in that direction, I just thought a little hint wouldn't hurt.