Worlds that serve a certain purpose and are not endless. Something like a blueprint, just on World level.
We could just define an area or discovered area in a world to be a world on its own
Great for quest lines for example.
But i imagine also something like hidden worlds within the regular worlds
I e. A hard-coded and size limited world is randomly generated within a regular world and can be accessed by certain events. Like passing a special portal, crafting such, reading hidden books, etc.
I imagine it like hell or nether in minecraft, just already pregenerated so it doesn't take too many resources.
And the ability for us players to add selfcreated subworlds to servers.
Examples might be a really huge ocean where people can do sea battles, a lava world with no resources or very specific ones. An empty space world to build space ships, etc
An advantage would be the no extra world generation us needed and such hidden things are awesome to find and explore