The question of terrain or level topology is easy as it is hard. I can make a level theme incredibly fast with the current data-driven engine, but somehow those themes don't satisfy me as they are. Levels need more details and also variations that should be difficult to predict. The slightly predictable level themes in Nethack (and other roguelikes) was something I wanted to fix with this game. But creating such levels is not easy.
Back a while I let go of strict data-driven approach and added special level themes, but at the same time I feel the data-driven code can do more and it already does when I added a special-ish level theme containing lots of lava. If however the data-driven system can't create all stuff needed in a level theme it has to be removed and used a manual generation instead. Hopefully most level themes can be created with the engine.
Even getting all level themes in some kind of physical form seems to be difficult, but it's something that should be done now, because you need levels to navigate in the game world and interact with game objects and since climate and other features play such an important role there must be different kind of themes to try out to see how easy or hard they are simply as terrain.