Create an expansion which goes underground and incoorporates features from a variety of the most beautiful caves in the world. For inspiration please take a look at some of the caves on this page. In addition, you should look at Carlsbad Caverns and Lurray Caverns in the U.S. for inspiration. Make sure the caves are well lit.
there is a cave in the Jungle Loop.
Maybe a large meteor crater or a diamond mine east of the new desert.
A lswift steep descent with a ong spiral up.
What about the bike park - mega cavern in Louisville, KY? https://louisvillemegacavern.com/attractions/mega-underground-bike-park