In the larger scale: realistic mountain chains (think Andes), valleys and terraces (Pamukkale), for example.
Also, smaller scale features such as rolling hills, river beds, features in a more “human” scale (World Machine defaults to large scale features like big mountains and I find it hard to do smaller features in it).
I’d be interested in pretty much everything related to the filters, combiners and selectors devices. Selectors could be a huge thing to expand and make interesting examples of.
Also a big focus on bitmaps for processing in external software like Unreal Engine. Especially for things like the vegetation and erosion devices (for example, to be able to generate procedural vegetation in Unreal by using World Machine exported bitmaps as a base to draw on, or texturing mountain sides using the talus output).
I’m not too interested in texturing for visualization inside World Machine, but exporting bitmaps that are usable in external tools is quite elusive when you have a complex terrain and not much understanding of how to select things properly.
Examples of game terrain that I find interesting: Horizon Zero Dawn and the Just Cause games (3 and 4).