Just like with mods and resource packs there should be a folder for schematics (buildings or whatnot) that could be selected for use in map generation. They would have adjustable parameters regarding location, frequency, and even random loot chests. Would be wonderful for random adventure map creation.

People could add new buildings to villages, ice castles in Ice Plains biome, Undersea labs or sunken ships at the bottom of oceans, possibly large cities, or even new hellish structures in the Nether. Over time the minecraft community would amass a vast library of content and players could simply select the ones they wish. Unique collections (groupings of various content by theme or whatever) could be select-able similar to seeds. The possibilities are endless.