Oh, I like this idea... even if toady restricts it to making cabbits
The main issue is that the hybrids can't be
created in the game "organically".
You can mod in a "half-elf" and it's a different race/creature-type, but there's no way to
make one in the game unless you just spawn a population of the "half-elf race", who then only breed with other half-elves. Just adding a "cabbit" race and
calling it a cat-rabbit hybrid, isn't
interesting at all in terms of game mechanics, or something Toady needs to help us with.
And changing the thing so that crossbreeds are
possible is in fact non-trivial because it challenges the entire conceptual level in a roguelike of the term "creature type" and any and all code that is
reliant on the concept of "creature type" being a non-mutable thing is now suspect and needs to be tested again or rewritten.
However, some middle ground
could work. If e.g. all races were defined as
creature variations from more general races, then you could create a taxonomy tree exactly like the real world. Then you could say that things within the "genus" level or "species" level could interbreed, and you have
procedural creature variations which mix and match the trait variations from each "species" definition of the creature variation, in defining which tags the
individual creature possesses (you'd also track which original "race" each inherited trait comes from - then you could quickly say "this person is X% human, Y% elf").
Then, e.g. dwarf and gnome could be in the same species ("dwarf-kind"), with elf, human, halfling defined as another species ("human-kind"). Both species would be part of a single genus. So "dwarf-kind" could mate with "human-kind" but they would have a low possibility of having fertile offspring (some tags could control this), while any of the "human-kind" can mate with each other. Then the "races" would be at a level
below species. e.g. "race" of human or elf is a
cultural distinction and someone who fits in neither one or the other would be called a "half-elf", but if you then mated with an elf and your offspring had enough elf-traits, then you'd just be considered an "elf".