Retro-Generates all biomes that have not been generated in yet. It is recommended to use /#remember beforehand to make sure the retro generation does not destroy player-created structures.
Be careful with this: Structures can cut into placer creations and similar. For a more precise retro-generation, use /#decorate.
You can also enter a Resource Expression as parameter to generate in all existing biomes, but just if the structures match the expression. This is useful for retro-generating dungeons into the existing world once they've been unlocked.