Swingly
The ability for worlds to have sub-instances to save on performance at the cost of a loading screen. Basically a portal or Udon function pointing to another world ID that creates a child instance, kinda like a linked list.
Moving to a child instance should of course have a prompt to avoid world creators using it as a void dimension for people they dislike. (maybe even add a button to return to the parent instance)
Rec Room has had a similar feature for a while, it allows for larger worlds without needing to fit everything in one scene. (especially useful for RAM-limited headsets like Quest)