What are you talking about here—I've been dreaming about it for ages for the Sims 4!
I'm primary the Sims 2 player, and the second rendition of the game has it so simple: you chose ANY world as a base world and than just add others as the districts.
I want to have an ability to create themed saves in the Sims 4: one save with the realistic urban worlds, one save with the realistic rural world, one save that contains only magical worlds, etc.
The ability to toggle on and off some worlds post factum (if I, for example, one day decide that my magical save needs an university) would also be cool.
And the most important thing: I want to see only the worlds I enabled on the world selection screen. I'm AuDHD, and all the worlds available in all the saves with no options to hide them is just too much for my brain, I can't play like that.
I'm literally cannot play the Sims 4 right now, I will hope some of my desired changes will be implemented in the future. Basically, I'm waiting for something like AnyGameStarter, but for the Sims 4.