Everyone needs to be sure to emphasize the city aspect of this idea, because otherwise you end up with generic castle/temple plus grass and vines and stuff, a simplistic theme that long before Nature's Ruins, JJ1 managed to do five times (Marbelara, Sluggion, Pezrock, Exoticus, and Stonar).

Another way to differentiate this from such sets: set the ruined city somewhere else. Instead of straight up grass and vines that everybody's seen many times before, why couldn't it get overtaken by ice or uranium? Sand's been done with Turtemple and Deserto and Temple and stuff, but there are other possibilities. Or it could be on an entirely alien world and have the encroaching nature look more like something out of sets like Slimy Land or Future.