In world of warcraft, the adjacent "tiles", which are pretty large, are loaded as a low poly model
This is done so you (as a player) do not see the "end of the world".
Then, once you know where the player is going, you have a Loading engine start to load the assets of the "tile" the player is going, asynchronously

Unload the once from the "tile" the player is moving from,

This "in short" is how Blizzard does it

Hope you can make something of it.

T.