Editing a set of terrains for an open world game

Hey guys, I want to have these tiled terrains (let's say 16x16 2048x2048 heightmaps) which I want to populate with a lot of props, trees, etc.
In game it's not a problem, as I can stream everything when needed, but how would I approach doing this in the editor, as it would all be in a single scene and I wouldn't be able to keep it all in memory at once or even render it.

Thanks!

 

Unity has the built-in Terrain system, but if you're going to create a big world you're going to quickly learn that it's (shortly put)... crap. Luckily for me I haven't had to use it much, but the fact that you can't make holes or overhangs in the terrain is quite limiting, so most people use 3rd party assets and tools.

That being said, why do you think you can't make the scene in the editor? Are you saying your computer won't be able to handle all the objects?