Perhaps a more general name would be "Gender Differentiation": the physical differences between the male and female gender in a given species. Yes, there was a topic about this previously, but it seemed to degenerate into arguments about human gender roles before dying a year ago. That's *us* humans, by the way, not DF "Humans".
Anyway.
You may have noticed that males and females of many species have notable differences between one another. Beyond the usual sex organs, there can be pronounced differences in the makeup and behaviour of the given genders. Males mammals are typically (but not always) stronger and more aggressive, or may sport extra natural weaponry (such as deer). In avian species, one gender or the other will be the 'attractive' one, trying to attract a mate with plumage or skill. In the case of one [extinct] species of bird, the male and female had different types of beaks to better work together in hunting and foraging.
Those are all real-world examples, and there's no pressure to directly copy anything into Dwarf Fortress. Perhaps a few conventional creatures could use an update (let's see how the hunter likes a deer turning his horns on them!), but as a user-editable fantasy universe there are numerous fun possibilities. Perhaps an ant-inspired monstrous insect species with common, weak males and uncommon but brutally powerful females? Or two competing civilizations of Dwarfs: one Matriarchal with tougher females, and the other Patriarchal with tougher males? Even these are mundane examples, translating gender into 'stronger' and 'weaker' roles. Why not have a species where the male and female are radically different, where they don't even have the same physical makeup? One flies, the other runs. One is an herbivore, the other is an omnivore.
Sexual dimorphism could be achieved in one of two ways: either the ability to separate a creature's raws into a 'male' and 'female' section to specify characteristics, or the ability for species to 'cross-pollinate': if two species can breed with one another in a prearranged way, then an enterprising modder can create a 'male' species and a 'female' species to jury-rig differentiation-- as well as create amusing hybrids for other animals. Certain new tags might be encouraged by the change. For example: a tag that changes the ratio of male to female births; or ones that dictate grouping arrangements (eg. [Monogamous]: species travels in mated pairs; [Pride]: species' males group together, females group together).
As much potential as there is for debate and discord, I think this'd be a fun addition to the game with much potential for new and interesting species. Any comments, criticism, or suggestions are welcome. If there are any format changes I should do (like make a succinct summary of the topic), let me know. Thanks.