This would be significant work. It's never as easy as just plopping a few assets around and calling it a day. Testing for glitches in terrain alone is a nightmare.
If there were to be new zones, we'd need a dedicated team to work on them first.
And if we're making new zones then we need to have a long, serious talk about the future of the story. Any new zone has to fit into the story. I like the Egypt idea. It doesn't further the story, it acts tangential and is rather unique.
We'd need modelers for Egypt themed things as well.
I dunno... its a great setting idea but the task would be monumental and if it were to be taken on, a huge amount of energy and dedication would be needed. No offense to the current dev team but I'm sure they know this; making content from scratch is always harder than it looks.