I really like the idea but it would be really difficult to execute this. The reason is not the map-design that would be as easy as it looks. The problem is letting the player design the actual surrounding of lots and the world itself. They would have to create an additional mode for the game with decorations, trees, rivers, mountains, fake buildings etc. etc.
That used to be easy in The Sims 2 because there weren't any complex surroundings but became really difficult yet manageable with The Sims 3, where you had to spend ages on a world especially when first starting. The problem with The Sims 4 is there is no overview for you to customize the world from above since the map is only just a picture. Of course the pictures of the maps look nice but then we'd have to replicate that in the world itself. They would have to find a way to let us customize the districts, the surroundings of those districts including places far away into the distance and then also the map. That would be a lot of work for them especially for a game that is already 6 years old.