I have a couple of quick ideas for how the "Hidden Fun Stuff" and/or the areas and z-levels leading up (down?) to it might be made more vibrant.
1. Rivers of Blood*. This is especially appropriate given the fact that, technically, the full title of the game is "Slaves to Armok, God OF BLOOD. Chapter 2: Dwarf Fortress". I would particularly like to see rivers of diseased (ie. syndrome causing) and heatproof (HEATDAM_POINT:NONE and BOILING_POINT:NONE) blood running through the bottom layer of hell, providing an area of (relative) safety for exploring adventurers.
2. On a similar note, it would be cool to have rivers and/or lakes of slime (a la Ghostbusters 2) appearing in the z-layers immediately preceeding the HFS. Lakes of slime can be done already, but I would like to have them cause a syndrome which causes those affected to go insane/berserk (something which there is no syndrome tag for as of yet) and, more importantly, as of yet there doesn't seem to be any way to limit them only to the levels close to the HFS. This leads me to my next topic.
3. To aid modding, there should be a token to limit stone types to appearing immediately above the DEEP_SURFACE and DEEP_SPECIAL layers, and another token to put them all the way at the bottom of the HFS.
* (Granted, technically, under the current system designate a custom inorganic substance using the blood material template instead of the stone template and set it so that veins (no pun intended) of the stuff appear in slade, but that still presents a number of problems, including (but not limited to): Its WAY too short. It doesn't flow. and it makes the slade too easy to breach.)