Lost Worlds is a companion mod for Lost Cities. It adds a new world preset that you can select during world creation and allows for five different types of worlds:

Islands: this is a void world with floating islands. In combination with Lost Cities you can populate these islands with cities (use the 'floating' profile for that)
Caves: this is a world similar to the nether but with regular stone and ores. You can use this in combination with the 'cavern' profile for Lost Cities. Warning! This is very hard. It may be recommended to use a spawn chest to help get started
Spheres: this is a void world that you can use in combination with the 'space' profile for Lost Cities. This will generate spheres which may contain cities
Normal: this is a normal style world that can be used in combination with the Lost Cities 'biosphere' profile. This is best used in combination with the 'Wastes' version of the world preset
Atlantis: this is a normal style world that can be used in combination with the Lost Cities 'atlantis' profile. The sea will be at height 90

More documentation on how to configure and setup this mod (for single player and multiplayer) can be found here: Lost Cities Wiki

To disucss this mod (and my others) you can visit my discord here: McJty Discord

This mod can be used freely in any modpack.