The only time they add a new world is when the new world enhances, shows off, or even offers new features of the pack.
Magnolia Promenade/Sixam: Added a place to build business lots, Sixam enhances the aliens themselves by adding a proper setting for them.
Windenburg: a world filled with residences part of clubs, the world itself has a lot to explore for clubs, a bunch of new locations to visit for clubs, dynamic social events take place for clubs, etc. the world was created with the whole concept of clubs in mind.
San Myshuno: The world itself was the biggest feature with a big chunk of the new gameplay confined to the actual world, a lot of the other major features are very well incorporated with the world itself, such as the new careers having certain requirements to visit locations and accomplish tasks within the new world.
Brindleton Bay: a world that has residences that own pets, the world includes a vet so that the player doesn't have to worry about placing one themselves, the world itself has a lot of gameplay opportunities for the pets themselves such as the bushes to explore, dig piles, birds to chase, places for cats to roam around, jump and climb, etc.
Granite Falls: the world itself was the biggest feature of the pack offering a new vacation setting.
Forgotten Hollow: a new dark grim location perfect for vampire residences, already has vampires living in the area, makes vampire training and socials available with no extra effort.
Selvadorada: the world itself is the biggest new feature of the pack with most of the new gameplay confined to that world.
Something like seasons doesn't necessarily need a new world to deliver its full set of features, a new world also wouldn't help expand or enhance the pack since seasonal climates and weather will be available across all the worlds. So to answer the question, I'm totally fine with not getting a new world with new packs if the theme of the pack doesn't really need a new world, let them put their resources in other areas of the pack to make the pack as a whole much better universally across the entire game.
