I really missed the custom world generation in previous Minecraft versions, and I was really happy to see it introduced again in 1.16. I used to love making randomized worlds and seeing if I could survive in a crazy bizarre landscape. So, since there was no out-of-the-box way to randomize worlds in the new version, I made my own tool.
Some features worth noting:
only tested on 1.16.1 at the moment;
generation is limited, so the worlds are not too glitchy (ie. low chance of sky to bedrock walls, etc.);
the Nether and End generation is currently left as default;
5% chance of a bedrock roof;
5% change of no bedrock floor (void);
20% chance of a lava ocean (though having no bedrock floor would remove this);
50% change of a 'bottom slide' that can create floating islands (though it pretty well makes it impossible to mine for some ores);
I'll leave the generation as is for the current minecraft version. If there's any breaking changes to the world generation in future versions (and I have the time), I'll introduce a version select and keep things for each version consistent: this means a 'seed' will generate the same output every time for a given version. So, if you get a really interesting generation and want to share you can just share the seed!
P.S. As a little 'how to': after you hit the "Download as JSON file" button in the tool and have your randomized JSON file, start creating a new world in minecraft, go to "More World Options..." -> "Import Settings" and open the downloaded JSON file through the "Select settings file" prompt that opens (it helps if you're not fullscreen when you do this). Hit yes to any prompts, don't change anything else in the "More World Options..", Create the world and Enjoy! Be warned that some worlds can be laggy.