So reading that, the image that came to mind was like one of those facebook games like Happy Acres or Royal Story or Farmville. But you want to make a 3d open world right?
You can see the entire expandible world, but can’t access it without buying/ gaining access to the expansions. Definitely look into level streaming. You could possibly have a 2d sprite background set up until the player gets close enough, then replaced with the actual 3d level.
You may have to bite the bullet and actually scale down everything as you and another mentioned earlier as well. The tricky part will be keeping that scale in W.M. too.
A possible combination of those could work, but its still a ton of data going through.
I played with this myself for a couple of hours. The trickiest part really is keeping everything lined up. I work in inch, ft, miles(as a southern American, it’s all i know lol). I made a large terrain that was 50x50 miles. Scaled it down to 50x50 feet. Scaled the textures the same way. Then I chopped it into (10) 5x5 sections. Each section is a new level. World composition has an origin shift option. Played with that a bit and char appears to still move over a couple of steps on lvl load.
Not sure its entirely possible atm with UE4. At least not without serious C++ scripts and some Engine mods.