In 1.23 you will be able to choose which block should be used to fill the flat terrain in creative mode. Before that it was always grass/dirt, but here’s the new options dialog in 1.23:
You can select a lot of different blocks, including glass or ice. If you have any special requests – give me a shout in the comments.
Here’s how the glass world looks like:
It’s quite disorienting to tunnel inside it, you can see all the surrounding passages. Contrary to expectations, it’s still dark if you make a deep enough cave in the glass. It’s because glass absorbs 1 unit of light, so 16 blocks below gets completely dark.
Here’s how a stone brick world looks like:
I hope this will make it easier for you to create unusual worlds.
More features are still to come in 1.23, including some new electric stuff.