Iirc, and I totally might not because I've been part of this community for like 12 years now, the last time I saw a thread on this topic both this particular suggestion of raw-defined hybrids linked to the parent species and modelling mixing directly were discussed. Raw-linked hybrids sounds easy, obviously, because all you have to do is make a system to properly link the critters. The procedural mixing of raw information to build hybrids on the fly is the hard part, but also exactly what Toady wanted to do from the beginning, and not linked directly to any of the development arcs that he's been working on over the last five or so years. But, it sounds like the perfect kind of thing for the myth and magic arc starting in the next year or so, where hybrid/generated playable races and monsters will appear more often in the world.
I would personally love to have either option, where I could define a hybrid I want to be used in a particular usage case (especially for Spellcrafts, where I would like certain night creatures to have origins in other species), and get procedurally generated hybrids on the side.
The basic structure of procedurally generated hybrids has to follow the rules of the raws themselves, where materials go into tissues which are used on body parts. A hybrid would have to start by combining the lists of materials and tissues of each of the parent species and then cross reference their body plans. If they both have quadruped_neck_hoof as the basic body plan (where the head, upper body and lower body are defined) then the choice is easy, but if one has quadruped_neck_hoof and the other is quadruped_neck (used for dogs and most quadrupeds that have feet/paws not hooves) then it has to choose, presumably a coinflip. Then the game should include every other body part that is shared by both parents, and coinflip on parts that are exclusive to one or the other. Then inherit the blood of one or both parents (it could be trying to merge a human and a spider for instance, where spiders bleed ichor but humans have blood) and decide if other additional tissue additions (like nails, hair, feathers, quills etc) apply based on which body parts it chose, i.e. if it chose quadruped with hoof, nails should be ommited because there isn't an appropriate body part to put them on. But if it combined a humanoid and a quadruped hoof body, making a centaur type thing, it should apply nails to the human fingers because it inherited finger body parts and the humanoid definition had nails on its fingers, but drop the reference to nails on the toes since it inherited hooves and has no appropriate body parts to put them on.
Mixing scales and skin becomes a problem though. If I chose to combine a crocodile and a donkey, and chose a donkey's body plan as the base, I would apply skin and body hair to everything inherited from the donkey and scales to everything inherited from the crocodile. The weird bit there is, the donkey has more body parts than the crocodile, and the crocodile has all of the body parts of a donkey minus a few iirc. So it would all end up with skin. But if it chose the crocodiles body as a base and covered it all with scales it could still inherit the donkeys ears, which the crocodile lacks, and thus put skin and fur on the ears, because they're donkey ears, even though almost every other part has scales.
But creatures where different castes have different body plans, like deer (where males have antlers and females don't) screw it up again. Do you choose to use the body plan from only caste? Or do you tell it to give the child race two different body plans for it's castes, taking attributes exclusive to the [MALE] parent and putting them on the [MALE] child and so on with the female. And what do you do with a creature that has more than two castes? Do you create the same number of castes for your child race? My Vutchnell creatures for instance have male and female versions of high, average and low castes (although they all have the same body plan, they have different physical and mental attributes and low Vutchnell lack the intelligence flag). If that definition was to be merged with elves, which have two castes, then should the child inherit the six caste structure and everything copied over from the Vutchnell castes' attributes, averaging the actual attribute differences against the elves, resulting in six castes of vutlves or whatever that are, high-averaged-with-elves, average-averaged-with-elves, and low-averaged-with-elves in terms of attributes and flags like the INTELLIGENT token.
But if handling multiple castes is too hard to program, and we do get a linked-raw-creature-hybridization, I can tell the game "a Vutchnell + a goblin = a gnoll" and it spit out gnolls whenever Vutchnell marry goblins.
(Or does my employer write me up for spending 45 minutes typing on my phone?)