I may have been too careful not to add too much content (like new creatures, items, etc.) during the process of programming the engine, but I guess it's time to forget the engine for a while and just add anything that is needed.
Level themes are quite strongly themed with specific monsters and other stuff, but I try to avoid making them too predictable by trying to break the rules randomly. I have thought that it could be possible to entirely replace a part of dungeon area with another theme, but that's something to consider later.
The problem with adding stuff is that it can be later tiresome to edit all that data if/when something changes, but then again I haven't spent all this time for nothing: the engine is flexible enough to introduce new stuff without too much extra programming.
So, again I'm at the snow top, wondering what kind of creatures live here...