I've found a good example that make worlds bigger is possible. Do you remember what maxis did when they realised The Sims Unleashed? They expanded existing world (in 2002). It means that in 2020 it is possible too especially that in The Sims 4 we also have no possibility to add our own lots in the world.
https://vignette.wikia.nocookie.net/sims/images/d/d8/Neighborhood_1.jpg/revision/latest?cb=20100121081730
https://vignette.wikia.nocookie.net/sims/images/6/67/Old_Town.png/revision/latest?cb=20140721071053