Going to a high school in Copperdale also goes through a loading screen, because it's a different lot. What you're asking for sounds more like an open world thing which, as we all know, doesn't seem possible for this game.
Also, I'm not sure if the developers are interested in removing already implemented worlds from the game (GTW career lots are technically their own "worlds", only hidden). I'm not sure how that would affect already existing saves. And yeah, they would have to remove these existing hidden career worlds if they wanted to "move" them to another world, even a brand new one.)
If you're asking for them to implement career lots to the map view, so to speak, than sure, I think they could go for that, but I don't know what that would accomplish. I guess one could build on them straight through manage worlds?
One also has to consider that the map views for Willow Creek and Oasis Springs would have to be adjusted for the career lots, and I don't even know if that is even possible to do in a way which doesn't affect people who do not own Get to Work.
And if I remember correctly, the developers did originally want Magnolia Promenade to be an addition to Willow Creek, but they could not get it to work without messing up existing saves, so they made it into its own world instead. Same story could be true for the career lots as well.
I do think, however, that there should be some way to easily enter build mode in these "hidden" lots.