I agree with you that certain parts of the world seem empty. But any one who plays mmorpgs knows this is sort of a thing. Its kind of an issue and kind of not an issue at the same time. It's just the way it is. As time goes on the game grows, every one wants to do the new bits and the old bits turn into a ghost town.

You can actually apply this same principle in real life. That's why there is a new car model every year. People want the new hotness and that's what drives sales.

It would be nice to see a fuller continents. There are many spots in existing old world map that could fit a new zone to fill in gaps in the map. Or convert some empty spots of existing zones to higher level areas to attract more players.

But we all know what's likely to happen is a new "hidden" continent/floating island/dimension will pop up long before any existing zone is retouched. Again, people want the new stuff and new stuff drives sales.