Something different instead of rehashed Forest, Plains, Snowy, etc. zones.
Howabout something different, like Shroom cities?
You can do a simply google search and see how varied underground zones could be. Anyone saying that an underground continent would be monotonous is simply being disingenuous.Most people hang around the main zones of the expansion. Which, in this case, are all Underground. I don't the entire expansion can center around Undermine. As a potential South Seas expansion, i think it would also involve places above ground like Plunder Isle or Tel Abim. You can't have a monotonous tone to the entire continent, as much as variety you can think of for Undermine.