What if your sim family lived in Newcrest and could only visit Magnolia Promenade, San Myshuno and Del Sol Valley? Or in a different save your sims lived in Strangerville, could only visit Oasis Spri...
Wasn't the big excuse for the new worlds being smaller that it's a problem for the game to keep track of so many of them? This could be a solution, I've thought about this too and I'd go for it.