We probably won't see that until after we see the first script-able procedural creature generators at the very least. Toady still needs to actually figure out a standard by which to structure whatever kind of script he comes up with for that.
If he comes up with one.
Which is why I'd like to bring the topic up from time-to-time. This whole thing would be much easier if he builds the procedural rules while keeping in mind the notion that he would want to make this raw-modifiable eventually, if not from the outset.
We have some control over how caverns are formed in the worldgen options, after all, so having some parameters for player control isn't unprecedented, here.
Raw-editable changes to the procedures for generating procedural creatures would be nice, and all, but changes to the way procedural citybuilding (or, say, procedural magic systems) takes place would have a much greater impact upon the ability for the player to really make their worlds feel different and reactive to the way they set their worlds up.
One of the major reasons I stopped modding DF was that I really couldn't create any sort of civ or creatures that weren't basically just humans or dwarves, because the game just isn't built to handle anything but dwarves with slight variations.