Two mods spring to mind:
The first one might be a bit difficult:
Name and flavor-text heritage mod:
Excepting "First of its kind" entities
1) Removes the second name in the series, leaving it blank. (Therefore, the name becomes just the first one which is Player changeable)
2) Removes the descriptive text, and sets the likes/dislikes to nothing
3) Removes the ability for artifacts to have a name, keeping them to just having a title (Ex: Frothyshadows "The Empty Slots of Fortune"= "The Empty Slots of Fortune")
4) Replaces the second name of living entities with the second name of its "First of its kind" ancestor, traced either through the female or the male line of ancestors based on whether their civ was first ruled by a Queen or a King - this becomes a family name (ex: Urist Smith - "Smith" is the family name).
5) Flavor text takes the values of its parent's and grandparent's appearances and also their likes/dislikes, averages them, then is re-populated in in the description, creating heritable physical and emotional values from one generation to the next. Emotional values to be modified by experiences with attacks, kidnappings, were-creature encounters and the like, such that those values will become more timid or more reckless/aggressive than their ancestor's averaged ones.
6) If a dorf inherits exactly the same values as both of his parents and grandparents (All four have identical body and emotional descriptions), then the emotion values are randomly valued and assigned instead.
When any new children are born, have this apply automatically to them as well.
With any luck, this might create easily traceable lineages in Legends mode, besides clearing up an unnecessarily cluttered entity list and simulate inherited genome patterns as well. With a bit of fiddling, a Mendelsonian dominant/submissive gene structure might be able to be worked out from these values too.
2nd Mod idea:
Food web mod:
Essentially, animal food webs are each created as independent creatures, with each individual creature in that food web as a caste of the overall creature. This way, a predator (pack or lone) is always encountered in an area where that predator's prey also happens to be, and creatures that wander into a Fortress aren't only a stream of one species at a time, but instead a regular population of hawk/snake/rabbit/mouse, for instance, which would make the simulation seem more "real".
(Incidentally, this could prove to be very interesting for the "giant" and "animal-person" versions of the food-web castes, with mixed populations providing special challenges for adventurers.)
The mod would aim for replacing all individual animal files with the biome-specific "Food-web" ones, which would keep polar bears from wandering around in tropical jungles and flocks of canaries from hanging out in the middle of the frozen tundra.