The idea is that you would be able to toggle more specific options during the world creation process.
All of the extra options would by default be set to false so if you just create a world it will be a normal vanilla world, but if you want to spice up the content inside your world you could go in the create tab and toggle the settings of the extra options to your liking. for example:
|extra animals| true or false,
if you toggle to true then you will have for example zebras, grizzly bears, sharks, etc. added to your world (The Alex's Mobs mod is a good example of extra vanilla animals that all have a use).
or another option could be
|harder end| true or false,
if you select true, then you could have to gather the eyes of the end in a different way for example killing the mini-bosses like the Elder Guardian Queen or the Wither and by exploring structures like the desert pyramids or jungle temples (The End Remastered mod is a nice example of this idea)
|more detailed terrain generation| true or false,
this would enhance the visuals of the vanilla biomes not add any new ones, for example, the trees would get a more randomized and natural look (an example of this could be the Dynamic Trees Mod or the Oh The Biomes You'll Go mod).
these options don't always have to be something rather big it could also be something like
|more skin textures for animals| true or false,
if it is set to true you would maybe have different cow chicken and pig textures or something like that.
This way Minecraft could add a lot more content to the game but you would still be able to play the very basic vanilla Minecraft, while on the other hand, you would be able to enhance your world in many different ways and combinations if there are enough togglable options.