The Sims 4 Producer Grant Rodiek said on The Sims Official Forums that expanding Neighborhoods and adding more of them in a single World is a possible thing!
When it comes to Loading Screens, Grant said that the frequency of Loading Screens is unlikely to be changed.
Larryad: Is it possible to at least open up the neighborhoods, so that you don’t have to incur a loading screen every time you enter a building within the neighborhood? I know that this has been stated before, but the worlds do need to be bigger. They’re tiny. Overall a great job on the Sims themselves though. I’d also like to see a Sim be able to flop down on a couch, the way they did in Sims 2. Also body hair for men please.
SimGuruGrant: We can potentially make the individual neighborhoods larger. We can also potentially add more neighborhoods to each world. The loading at this point is a fundamental part of our architecture and is unlikely to change.
