One thing I have found interesting is that they replaced custom world generation with Buffet, making it harder to try different world settings. Also with Buffet, there should be a setting to make it so multiple biomes can appear and even specially set biome mixes,like a dry world, or ice land, and give better access to customized worlds without mods, or add-ons. The customization tab should be brought back, and with the new Village and Pillage update(1.14) it could allow for very special setups that could be shared throughout the minecraft community,and give ideas for better things.
(note:this could also be used to make minecraft realms and could have a section dedicated to adding extra structures for worlds)
Quick thought, this could allow for you to add structures made in test worlds and put them as natural spawning in custom worlds.this could be useful in non modded structures